[in_app_purchase] Update typo in example main.dart

[LewisHobden]

Documentation typo.

Description

Fixed a typo in an inline comment.

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[LewisHobden]

@googlebot I signed it!

@googlebot I signed it!

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CLAs look good, thanks!

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[LHLL]

Thanks for the PR! LGTM.

[LHLL]

Sorry, can you also update CHANGELOG.md and pubspec.yaml?

[LewisHobden]

I have committed changes to CHANGELOG + pubspec.yaml!

[LHLL]

Thanks a lot!