[docs] rllib-models.rst - QMIX +parametric (#8868)

Updating docs to show that QMIX supports parametric action space, as per SMAC environments. 

This is reflected in the code here: https://github.com/ray-project/ray/blob/master/rllib/agents/qmix/qmix_policy.py#L179 and consistent with QMIX being an extension of DQN

doc/source/rllib-algorithms.rst

@@ -22,7 +22,7 @@ Algorithm           Frameworks Discrete Actions        Continuous Actions Multi-
 `MARWIL`_           tf + torch **Yes** `+parametric`_  **Yes**            **Yes**     `+RNN`_
 `PG`_               tf + torch **Yes** `+parametric`_  **Yes**            **Yes**     `+RNN`_, `+autoreg`_
 `PPO`_, `APPO`_     tf + torch **Yes** `+parametric`_  **Yes**            **Yes**     `+RNN`_, `+autoreg`_
-`QMIX`_             torch      **Yes**                 No                 **Yes**     `+RNN`_
+`QMIX`_             torch      **Yes** `+parametric`_  No                 **Yes**     `+RNN`_
 `SAC`_              tf + torch **Yes**                 **Yes**            **Yes**
 ------------------- ---------- ----------------------- ------------------ ----------- ---------------------
 `AlphaZero`_        torch      **Yes** `+parametric`_  No                 No