fix: duplicate step number 2 in custom robot instructions

[andrewwhitecdw]

This PR corrects the documentation in README.md: duplicate step number 2 in custom robot instructions.

Changes

• README.md: duplicate step number 2 in custom robot instructions.

Details

Before:

2. Implement the ``build()`` method.  This method is responsible for adding the robot to the USD stage.
2. Implement the ``write_action()`` method.  This method performs the logic of applying the linear, angular velocity command.

After:

2. Implement the ``build()`` method.  This method is responsible for adding the robot to the USD stage.
3. Implement the ``write_action()`` method.  This method performs the logic of applying the linear, angular velocity command.