Replace grpo_config with algo_config while calling GRPOLearner in GRPO Demo notebook

[rajasekharporeddy]

With the commit 40c2577, the grpo_config argument has been renamed to algo_config in GRPOLearner.

However, it was not modified in GRPO Demo (examples/grpo_gemma.ipynb) notebook which results in a TypeError.

TypeError: GRPOLearner.__init__() got an unexpected keyword argument 'grpo_config'

Also, this PR removes the description of grpo_config in the docstring of the GRPOLearner class.