Update documentation

/cc @accolade

README.md

@@ -104,7 +104,7 @@ Plug 'junegunn/fzf', { 'dir': '~/.fzf', 'do': './install --all' }
 " Unmanaged plugin (manually installed and updated)
 Plug '~/my-prototype-plugin'
 
-" Add plugins to &runtimepath
+" Initialize plugin system
 call plug#end()
 ```
 

plug.vim

@@ -40,7 +40,7 @@
 "   " Unmanaged plugin (manually installed and updated)
 "   Plug '~/my-prototype-plugin'
 "
-"   " Add plugins to &runtimepath
+"   " Initialize plugin system
 "   call plug#end()
 "
 " Then reload .vimrc and :PlugInstall to install plugins.