Update usage examples

vim-plug has no support for dependencies. Remove comment that has been
a source of confusion. #544

plug.vim

@@ -18,7 +18,7 @@
 "   " Any valid git URL is allowed
 "   Plug 'https://github.com/junegunn/vim-github-dashboard.git'
 "
-"   " Group dependencies, vim-snippets depends on ultisnips
+"   " Multiple Plug commands can be written in a single line using | separators
 "   Plug 'SirVer/ultisnips' | Plug 'honza/vim-snippets'
 "
 "   " On-demand loading