Fix links and pyglet version

[dan-pandori]

• Redirected ppo2 to ppo, since the former seems not to exist in that docs page.
• Redirect to the gymlibrary docs for CartPole, since the old link failed.
• Added pyglet==1.4 to the list of required packages to install, since one of the later cells needs this.

Tested by running all cells from the following Google colab link:
https://colab.research.google.com/github/dan-pandori/rl-tutorial-jnrr19/blob/linkfix/1_getting_started.ipynb

And verifying that the edited links went to reasonable locations.

[araffin]

LGTM, thanks for the fixes =)

[dan-pandori]

Thanks for setting up the tutorial! It's been very helpful