DOC: fix some sphinx build issues
The table here: http://pandas-docs.github.io/pandas-docs-travis/categorical.html#concatenation is apparently not building well. Rst ...

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14332?src=pr) is 85.25% (diff: 100%)

> Merging [#14332](https://codecov.io/gh/pydata/pandas/pull/14332?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14332   diff @@
==========================================
  Files           140        140          
  Lines         50630      50630          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43169      43167     -2   
- Misses         7461       7463     +2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6dcc238...0419321](https://codecov.io/gh/pydata/pandas/compare/6dcc23862b6b60ce2a67436b4a278fbe4c05490f...0419321c8859d21e0a8c22bedcb29581756d4543?src=pr)
