DOC: Fixed a minor typo
It also seems that the texts of the `doc/README.rst` and the `doc/source/contributing.rst` have lots of overlap and duplication. Not sure if there are any plans in consolidating these two files but I thought to mention that here 😄 

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13366?src=pr) is **84.23%**

> Merging [#13366](https://codecov.io/gh/pydata/pandas/pull/13366?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13366   diff @@
==========================================
  Files           138        138          
  Lines         50724      50724          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42726      42726          
  Misses         7998       7998          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [103f7d3...029ade7](https://codecov.io/gh/pydata/pandas/compare/103f7d31e1b850e532ed85a4b53ef222d1271c54...029ade7c9fd65da98e22c09c4e7911e80be95cc5)
 thanks.

the README is for 'local-use' IOW its there waiting to be read if needed. the contributing.rst is built into the published docs.
 Cool @jreback. I hope I can contribute more to this awesome library 😄 
