Enable fuzzy-matching for helm-projectile-find-file

Helm now supports fuzzy matching by simply enable :fuzzy-match slot in
helm-build-in-buffer-source function. The function is currently the
idiomatic way to create source with candidates in buffer with Helm. We
must use this function to enable fuzzy matching, with candidates in
buffers (as candidates-in-buffer is being used by Helm Projectile
everywhere).

Also fix use proper help message function: 'helm-ff-help-message instead
of 'helm-find-file-help-message.