docs: sync mypy command in GEMINI.md with CI

In [CI](https://github.com/a2aproject/a2a-python/blob/3ad2503107dd1faa71ab6b56da9017141e24e34c/.github/workflows/linter.yaml#L41) we don't run mypy on tests, so in order to prevent model from encountering old issues specify the same command as in CI.

Author: ishymko

GEMINI.md

@@ -12,7 +12,7 @@
    ```
 3. Run mypy type checkers to check for type errors
    ```
-   uv run mypy
+   uv run mypy src
    ```
 4. Run the unit tests to make sure that none of the unit tests are broken.
   ```