Update ros2-gazebo.rst to include sudo for directory write permissions

[mueller-lukas]

This pull request addresses write permission issues in the documentation by adding sudo to necessary commands in the ros2-gazebo.rst file. Specifically, it modifies commands related to adding Gazebo APT sources and updating Gazebo sources in rosdep to ensure they can be executed without permission errors. The changes include:

Added sudo to tee command for writing to /etc/apt/sources.list.d/gazebo-stable.list.
Added sudo to wget command for writing to /etc/ros/rosdep/sources.list.d/00-gazebo.list.

These changes ensure that users can follow the documentation without encountering write permission issues.