DOC: greater consistency and spell-check for intro docs
I read through introductory docs, and made the following changes:

- Added missing periods, which were sometimes absent in short sentences.
- In other parts of the documentation, code examples appear to typically be introduced with a colon. This has been added in some sentences which introduce code and do not end in a period nor a colon.
- Updated a reference to the Python 2 docs to Python 3.
- Restructured a few sentences slightly.
- Found an example of code that did not run due to an extra `}`, which I removed.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18948?src=pr&el=h1) Report
> Merging [#18948](https://codecov.io/gh/pandas-dev/pandas/pull/18948?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ee9c7e9b19a1fa3c4499d10885dec72f1d651832?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/18948/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18948?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18948      +/-   ##
==========================================
- Coverage   91.59%   91.57%   -0.03%     
==========================================
  Files         150      150              
  Lines       48959    48964       +5     
==========================================
- Hits        44845    44838       -7     
- Misses       4114     4126      +12
```

| 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/18948?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18948/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/18948/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.08% <0%> (ø)` | :arrow_up: |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/18948/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.36% <0%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18948?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18948?src=pr&el=footer). Last update [ee9c7e9...e8cb0f3](https://codecov.io/gh/pandas-dev/pandas/pull/18948?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback and @jschendel : I have made the requested changes. thanks @tommyod 

built docs are: http://pandas-docs.github.io/pandas-docs-travis/ (likely take a few hours). please make sure changes are rendered ok. thanks! @jreback - Great, will do. I hope to find the time to read more of the documentation and make similar changes if I see room for improvement, i.e. punctuation, function references, small changes to sentences, and so forth.

If so, what is the sensible size of a PR? Should I create a PR for every file, every 3-4 files in the documentation, or wait and submit a PR when I have read and made changes to most of the docs? Any preference? @tommyod : I don't think there's really a preference in terms of the size of a PR.  Whatever works best for you should be fine.  My advice is to lean towards submitting PR's more frequently as opposed to waiting to submit a huge number of changes at once.  Waiting could lead to merge conflicts if other people are updating the same sections of the docs, which could lead to extra work resolving the conflicts. Thanks for improving the docs!