[vim] Use `pathogen#infect('plugins/{}')` in vimrc

Change `pathogen#infect('plugins')` to `pathogen#infect('plugins/{}')`
as the syntax for specifying a custom directory for plugins has changed.

Ref: https://github.com/tpope/vim-pathogen/tree/b9fb0dfd811004010f5f6903edef42d6004ebea2#readme