Make various minor corrections to cheatsheet
Changes made: 

 - Fixed SubHeading `Make New Variables` to `Make New Columns` -- because these operations create new columns.
 - Fixed # of distinct values in a column to use the pandas method `.nunique()` rather than `len(df['w'].unique())`.
 - For clarity, renamed the `Length` column selection to `w` under the Summarize Data -- can see this being misleading to someone thinking that to get "Count number of rows with each value of the variable" they might need to call `Length`. 
 - For legibility/clarity removed the times that the example df was renamed (instances of `df=df.dropna()`); we should perhaps make a higher-level note that all of these operations are by default not done `inplace`, and either need to reassign or `inplace=True`.

Updated .pptx and .pdf
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=h1) Report
> Merging [#16136](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/186957ef153adaba367ffcd5cb0162488416b59f?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16136/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16136      +/-   ##
==========================================
- Coverage   90.86%   90.83%   -0.03%     
==========================================
  Files         159      159              
  Lines       50773    50773              
==========================================
- Hits        46134    46122      -12     
- Misses       4639     4651      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.61% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.33% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.54% <0%> (-1.82%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=footer). Last update [186957e...aaa04f3](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=h1) Report
> Merging [#16136](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/186957ef153adaba367ffcd5cb0162488416b59f?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16136/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16136      +/-   ##
==========================================
- Coverage   90.86%   90.83%   -0.03%     
==========================================
  Files         159      159              
  Lines       50773    50773              
==========================================
- Hits        46134    46122      -12     
- Misses       4639     4651      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.61% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.33% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.54% <0%> (-1.82%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=footer). Last update [186957e...aaa04f3](https://codecov.io/gh/pandas-dev/pandas/pull/16136?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. @jorisvandenbossche  @jbwhit thanks!