Fix naming of buffers for tags ending in Y in 'mpc'

Fix naming of buffers for tags ending in Y in 'mpc'

* lisp/mpc.el (mpc-tagbrowser-buf): Use 'mpc-tagbrowser-tag-name'
to ensure buffers corresponding to tags that end in Y are
correctly pluralized; e.g. the tag "Directory" should create a
buffer named "*MPC Directories*" not "*MPC Directorys".

(cherry picked from commit 60e58be091bf075a03ac431444aa9c9b5832bb19)