docs cheat sheet pandas Pd to pd
Change pandas syntax from Pd to pd

Closes [issue 15017](https://github.com/pandas-dev/pandas/issues/15017#issuecomment-269871137)
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15026?src=pr) is 84.77% (diff: 100%)
> Merging [#15026](https://codecov.io/gh/pandas-dev/pandas/pull/15026?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #15026   diff @@
==========================================
  Files           145        145          
  Lines         51131      51131          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43345      43345          
  Misses         7786       7786          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3662413...3cca5d2](https://codecov.io/gh/pandas-dev/pandas/compare/366241386282272328333256b6e267a68f8133a4...3cca5d28c5bebdb0e86e63af8423e4a119e9f5b9?src=pr) thanks!