fix warning box in readme

README.md

@@ -79,7 +79,7 @@ activate-global-python-argcomplete
 eval "$(register-python-argcomplete ros2-pkg-create)"
 ```
 
-> **Warning**  
+> [!WARNING]  
 > Outside of a virtual environment, *pip* may default to a user-site installation of executables to `~/.local/bin`, which may not be present in your shell's `PATH`.  If running `ros2-pkg-create` errors with `ros2-pkg-create: command not found`, add the directory to your path. [*(More information)*](https://packaging.python.org/en/latest/tutorials/installing-packages/#installing-to-the-user-site)
 > ```bash
 > echo "export PATH=\$HOME/.local/bin:\$PATH" >> ~/.bashrc