[RLlib] Typo in error message on line 37 of ray/rllib/env/utils/__init__.py

[MatthewCWeston]

Description

Currently, it says:

 raise ImportError(
                "Could not import pyspiel! Pygame is not a dependency of RLlib "
                "and RLlib requires you to install pygame separately: "
                "`pip install pygame`."
            )

This appears to be a copy-paste error. It should say:

 raise ImportError(
                "Could not import pyspiel! Pyspiel is not a dependency of RLlib "
                "and RLlib requires you to install pyspiel separately: "
                "`pip install open_spiel`."
            )