Fix some grammatical mistakes

[pycub]

• Because this PR includes a bug fix, relevant tests have been included.
• Because this PR includes a new feature, the change was previously discussed on an Issue or with someone from the team.
• I didn't do anything of this.

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This PR proposes three grammatical enhancements:

1. on tutorial -> in tutorial. reference.
2. Oxford comma.
3. Unnecessary use of then, and missing ,.

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