Unit 1 notebook small writing improvements

[DennisSoemers]

This PR just includes a few (really minor) writing improvements in the notebook for Unit 1.

Note: the most impactful change is probably that I suggest to remove this (line 529):

"- The game is solved if your agent does 200 points."

Unless I missed it, the rest of the course (at least so far, so just with Unit 1) does not get into details anywhere on the idea of "solving" when passing a certain threshold in gym environments. It's also really uniquely a gym thing though, and not a concept that applies anywhere across the rest of RL, so in my opinion it's better to not mention it at all.

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[simoninithomas]

Hi Dennis.

Thanks for the updates. I'm merging it. I'm currently updating each of the colab so your feedback is very useful 🤗 . I merge it.

I'm agree with the line update ""- The game is solved if your agent does 200 points."" I think I will replace with: for this exercise try to reach ~200 mean_reward.