Clarify that function call requires `call`

Close #487

README.md

@@ -62,9 +62,9 @@ $uri = 'https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
 
 Add a vim-plug section to your `~/.vimrc` (or `~/.config/nvim/init.vim` for Neovim):
 
-1. Begin the section with `plug#begin()`
+1. Begin the section with `call plug#begin()`
 1. List the plugins with `Plug` commands
-1. `plug#end()` to update `&runtimepath` and initialize plugin system
+1. `call plug#end()` to update `&runtimepath` and initialize plugin system
 
 #### Example