Rename script-local function to generate better error message

This commit renames s:add which is called on Plug command to s:Plug.

E116: Invalid arguments for function <SNR>2_Plug

Related: #407

plug.vim

@@ -128,7 +128,7 @@ function! plug#begin(...)
 endfunction
 
 function! s:define_commands()
-  command! -nargs=+ -bar Plug call s:add(<args>)
+  command! -nargs=+ -bar Plug call s:Plug(<args>)
   if !executable('git')
     return s:err('`git` executable not found. Most commands will not be available. To suppress this message, prepend `silent!` to `call plug#begin(...)`.')
   endif
@@ -458,7 +458,7 @@ function! s:lod_map(map, names, prefix)
   call feedkeys(a:prefix . substitute(a:map, '^<Plug>', "\<Plug>", '') . extra)
 endfunction
 
-function! s:add(repo, ...)
+function! s:Plug(repo, ...)
   if a:0 > 1
     return s:err('Invalid number of arguments (1..2)')
   endif