Fix: bad function usage in README for UltiSnips (#228)

This commit fixes an issue with using vsnip#anonymous in the snippet expand function for Ultisnip
The main issue, is that when using vsnip#anonymous instead of UltiSnip#Anon when calling the cmp.mapping.confirm map on a non-snippet option would "crash" cmp, ceasing its normal functionality. Using UltiSnps#Anon fixes this issue.

README.md

@@ -73,7 +73,7 @@ lua <<EOF
         -- require('luasnip').lsp_expand(args.body)
 
         -- For `ultisnips` user.
-        -- vim.fn["vsnip#anonymous"](args.body)
+        -- vim.fn["UtiliSnips#Anon"](args.body)
       end,
     },
     mapping = {