DOC: Fixed minor spelling errors
Two small changes:

- Two minor spelling errors corrected.
- Added part 7 of a tutorial series to the docs (parts 1-6 were included already, but not part 7).

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18941?src=pr&el=h1) Report
> Merging [#18941](https://codecov.io/gh/pandas-dev/pandas/pull/18941?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cdebcf374598e9141df7dc23bc75c9528e92c004?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/18941/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18941?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18941      +/-   ##
==========================================
- Coverage   91.59%   91.57%   -0.03%     
==========================================
  Files         150      150              
  Lines       48959    48959              
==========================================
- Hits        44843    44833      -10     
- Misses       4116     4126      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.93% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.13% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18941?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18941/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18941/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.9% <0%> (+0.21%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18941?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/18941?src=pr&el=footer). Last update [cdebcf3...30b5b97](https://codecov.io/gh/pandas-dev/pandas/pull/18941?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks! Thanks for merging @jorisvandenbossche. I plan to read the docs carefully. If I find similar errors I will submit a PR.