DOC: Add "nullable float" link in "dtypes" table of User Guide
The formatting of the "nullable float" entry in [this table](https://pandas.pydata.org/docs/user_guide/basics.html#dtypes) was inconsistent with the other entries. As far as I can tell, there is not (currently) a "nullable float" section of the User Guide, so I've added a reference to the API, as is the case for the table's "Boolean (with NA)" entry.
Thanks @dpoznik 