DOC: Minor correction to DataReader data_source options
Documentation for DataReader had noted that Fama/French was "ff" for data_source, rather than "famafrench". 

Modified DataReader slightly to accept "ff" anyway.

this needs a test. caveat: you would need to write the `DataReader` test suite since one does not currently exist. i'm cleaning up `data.py` and i've added a bare bones test suite for `DataReader`, so i can merge this after the cleanup..or can merge this first and add to the new test suite...@jreback?
 If you want to merge your test suite for DataReader, I can pull that and update it to include a test for the "ff"/"famafrench" option.
 #4047 ... i'll wait till it passes then merge, then u should rebase and then we can merge your pr
 also please add a blurb to release notes (they are in `doc/source/release.rst`) and copy it to `doc/source/v0.12.0.txt` maybe under the "Enhancement to existing features" heading(s)
 I won't have time until next week to get more into this commit, but this should get us started. 

I think it's good enough for this PR, and more advanced testing (i.e. value checking) should probably be another tracked issue.
 I'm going to close this one for now, I'll reopen a PR with the changes after the dust settles on data.py. Cheers!
