Replace 'Bundle' with 'Plugin' in Vundle docs

This reflects recent changes to Vundle:
https://github.com/gmarik/Vundle.vim/commit/bf70a158c5db74b5c1415713976d41943788d0d1
Merge pull request #48 from c089/replace-bundle-with-plugin

Replace 'Bundle' with 'Plugin' in Vundle docs