doc: fix typo in generator.md

[techtaco86]

This is a minor typo fix, but it was bothering me too much...

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Fixes #<issue_number_goes_here> 🦕

[techtaco86]

@danielbankhead @sofisl It seems I can't request a review directly ?

[techtaco86]

thanks @d-goog I wasn't sure who to ping for review.