DOC: Fix grammar and formatting typos 
One grammatical typo, and the rest address small formatting typos when rendering online.  
@mroeschke Thanks for the PR! Just one comment about the line length (our strict rules nowadays are sometimes a bit cumbersome for the docstrings ..) Ah you were correct; Travis threw a linting error. Thanks for the tips and explanation about #noqa! I'll fix those tonight.   ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14803?src=pr) is 85.28% (diff: 100%)
> Merging [#14803](https://codecov.io/gh/pandas-dev/pandas/pull/14803?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14803   diff @@
==========================================
  Files           144        144          
  Lines         50968      50968          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43466      43467     +1   
+ Misses         7502       7501     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [2466ecb...e08d27e](https://codecov.io/gh/pandas-dev/pandas/compare/2466ecbb717d8cdfd30cc20d5d22e5e095d9a14d...e08d27e578c6c95ed7ba019c7a1199b60dcc599d?src=pr) @mroeschke the linter is now complaining about some other lines:

```
pandas/core/generic.py:3638:80: E501 line too long (84 > 79 characters)
pandas/core/generic.py:3646:80: E501 line too long (80 > 79 characters)
pandas/core/generic.py:3660:80: E501 line too long (80 > 79 characters)
pandas/tools/plotting.py:2896:80: E501 line too long (80 > 79 characters)
```

(also lines that you didn't change, but I suppose that the `#noqa` suppressed them as well) Oops. Thanks for the heads up. I'll address those tonight as well.  Hmm `git diff master | flake8 --diff` and `pep8radius master --diff` passed locally, but it seems Travis still threw a linting error? @jorisvandenbossche 

 Yep (you can always look at the travis log (third build): https://travis-ci.org/pandas-dev/pandas/jobs/181861282, scroll to bottom):

pandas/core/generic.py:3662:80: E501 line too long (80 > 79 characters)

It's from lines that you didn't touch yourself (the full docstring was not checked before), so checking the diff didn't catch this. BTW, besides checking the diff manually, I would also advice to set up your IDE to check for this.
 Ah, thanks for the guidance! Still learning to navigate myself through the logs. I appreciate the help! @mroeschke Thanks!