keymap validation: Improve message for action that takes no input (#37877)

See
https://github.com/zed-industries/zed/issues/26370#issuecomment-3200022302

Before (user screenshot):

<img width="650" height="127" alt="Image"
src="https://github.com/user-attachments/assets/9548ce43-657e-46ef-b6be-b95489167ac2"
/>

After (my screenshot):

<img width="616" height="229" alt="image"
src="https://github.com/user-attachments/assets/09028305-e7bb-4a31-bb87-55effbec26f3"
/>

Release Notes:

- N/A

docs/src/vim.md

@@ -490,7 +490,7 @@ Vim mode comes with shortcuts to surround the selection in normal mode (`ys`), b
 {
   "context": "vim_mode == visual",
   "bindings": {
-    "shift-s": ["vim::PushAddSurrounds", {}]
+    "shift-s": "vim::PushAddSurrounds"
   }
 }
 ```