Update readme

README.md

@@ -166,10 +166,10 @@ Note: calling the setup function is only required if you wish to customize this
 
 ```lua
 require("cmp_nvim_ultisnips").setup {
-  filetype_source = "treesitter",
+  filetype_source = "ultisnips_default",
   show_snippets = "all",
   documentation = function(snippet)
-    return snippet.description
+    return snippet.description .. "\n\n" .. snippet.value
   end
 }
 ```