Use default_capabilities() in doc and util (#1254)

Commit 0b6654d3de changed the `README.md`
to use `default_capabilities()` instead of `update_capabilities()`. This
commit replicates that change to `doc/cmp.txt` and `utils/vimrc.vim` for
consistency.

utils/vimrc.vim

@@ -44,7 +44,7 @@ cmp.setup {
 EOF
 
 lua << EOF
-local capabilities = require('cmp_nvim_lsp').update_capabilities(vim.lsp.protocol.make_client_capabilities())
+local capabilities = require('cmp_nvim_lsp').default_capabilities()
 
 require'lspconfig'.cssls.setup {
   capabilities = capabilities,