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.

doc/cmp.txt

@@ -126,7 +126,7 @@ A recommended configuration can be found below.
     })
 
     -- Setup lspconfig.
-    local capabilities = require('cmp_nvim_lsp').update_capabilities(vim.lsp.protocol.make_client_capabilities())
+    local capabilities = require('cmp_nvim_lsp').default_capabilities()
     require('lspconfig')[%YOUR_LSP_SERVER%].setup {
       capabilities = capabilities
     }