update phf::PhfMap to phf::Map

phf public interface updated in commit:
sfackler/rust-phf@7fc934a23e7e25fd12014a123eea8f7707928338
update generator usage doc to reflect phf::PhfMap -> phf::Map change
Merge pull request #12 from DanielFerguson/fix-phfmap-naming

update phf::PhfMap to phf::Map