DOC: Remove alias for numpy.random.randn from the docs
Based on this comment: https://github.com/pandas-dev/pandas/issues/28038#issuecomment-523439376

Supersedes #28068 
@datapythonista Hi Marc, I've done this do you mind having both changes in the same PR? thanks @WuraolaOyewusi  ok @datapythonista  @WuraolaOyewusi Thanks a lot!