Fix typos in continuous mountain-car documentation

[schillic]

Description

This PR fixes two typos in the documentation of the continuous mountain-car environment.

It can be seen in the discrete version (and is common sense) that the dynamics of $v_{t+1}$ should not mention $v_{t+1}$.

Gymnasium/gymnasium/envs/classic_control/mountain_car.py

Line 60 in 3287c86

*velocity<sub>t+1</sub> = velocity<sub>t</sub> + (action - 1) * force - cos(3 * position<sub>t</sub>) * gravity*

Type of change

Please delete options that are not relevant.

• Documentation only change (no code changed)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes