Update vim commenting shortcut to `cc`

Now that `<space>` is leader, it is impractical to type `<space>,`. It turns out
that `cc` is a synonym mapping for `S`, and since `S` is easier to use (and has
been used), `cc` has been taken over for single-line and visual commenting.

vim/settings/nerdcommenter.vim

@@ -1,3 +1,3 @@
 " Toggle Commenting out lines with NERDCommenter
-nnoremap <silent> <Leader>, :call NERDComment("n", "toggle")<CR>
-vnoremap <silent> <Leader>, <Esc>:call NERDComment("x", "toggle")<CR>
+nnoremap <silent> cc :call NERDComment("n", "toggle")<CR>
+vnoremap <silent> cc <Esc>:call NERDComment("x", "toggle")<CR>