DOC: Fix typo in genetic code docstring

Fixes #536.
Merge pull request #539 from jairideout/issue-536

DOC: Fix typo in genetic code docstring
ENH: added test based on suggestion from @ebolyen
Merge branch 'master' of github.com:biocore/bipy into issue-537
REL: added note on changes based on suggestion from @ElDeveloper