README: fix typo to match Makefile: s/PREFIX/prefix/

Signed-off-by: Emilio G. Cota <cota@braap.org>
README: fix missing mention to 'prefix' flag for `make'

Signed-off-by: Emilio G. Cota <cota@braap.org>
Merge pull request #40 from cota/master

README fixes