Added missing install to pnpm (#1355)

## 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)
- [ ] I have not broken the cheatsheet

Author: AndreasArvidsson

docs/contributing/CONTRIBUTING.md

@@ -21,7 +21,7 @@ extension](#running--testing-extension-locally), you may want to check out the
 3. Run the following in the terminal:
 
    ```bash
-   pnpm
+   pnpm install
    pnpm compile
    ```