Correct operator examples in Stage 0 documentation

[slowpoke111]

Description

Fixes 3 minor inaccuracies in Stage 0's operators lesson to ensure that the page is consistent with itself and teaches proper java.

What changed? Why? (the code + comments should speak for itself on the "how")

1. The remainder row in the Operators table has had its example updated from a/b to a%b
2. Updated the answer to the 2nd question to clarify that magicNumber's value is not reassigned.
3. Updates the += example's description to match what the code does.
   We highly recommend including screenshots or a video of what was changed, to collect evidence in a place where we can later reference it.

Closes #135

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why, including events that led to this PR
• Pull Request has been linted and formatted and follows the styleguide

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