Fix custom actions in cheatsheet (cursorless-dev#1973)

- Fixes cursorless-dev#1960



## Checklist

- [-] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [-] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [x] I have not broken the cheatsheet

src/actions/actions.py

@@ -45,6 +45,7 @@
     "reformat_action",
     "call_action",
     "experimental_action",
+    "custom_action",
 ]
 
 callback_actions: dict[str, Callable[[CursorlessTarget], None]] = {