Use more appropriate action for Vim word completions (#28043)

Follow-up of https://github.com/zed-industries/zed/pull/26410

The action does not sort the items the way Vim does, but still better
than the previous state.

Release Notes:

- N/A

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

assets/keymaps/vim.json

@@ -354,8 +354,8 @@
   {
     "context": "vim_mode == insert && !(showing_code_actions || showing_completions)",
     "bindings": {
-      "ctrl-p": "editor::ShowCompletions",
-      "ctrl-n": "editor::ShowCompletions"
+      "ctrl-p": "editor::ShowWordCompletions",
+      "ctrl-n": "editor::ShowWordCompletions"
     }
   },
   {