correct the error msg for invalid env registering

[ashwinsnambiar]

Why are these changes needed?

If a custom env is used and it is not registered properly, the ERR_MSG_INVALID_ENV_DESCRIPTOR error message from rllib.utils.error.py is raised. The message is incorrect as it tells the user to use tune.register() and use the old config(env) to set the env in config.

Related issue number

Closes #51620

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    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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