Update installation.rst
Update installation.rst

Minor fix—match text to linked filename. Perhaps it would be better not to include the version number in this line; it will need to be updated every time to match the link and 'alpha version of the NLTK' is informative enough on its own.

Former-commit-id: 19f103a5185530e5b2a969b532f437fc77672d38
marpozzi/cltk,TylerKirby/cltk,D-K-E/cltk,LBenzahia/cltk,mbevila/cltk,diyclassics/cltk,TylerKirby/cltk,eamonnbell/cltk,cltk/cltk,kylepjohnson/cltk,coderbhupendra/cltk,LBenzahia/cltk