[in_app_purchase] fix dead links

[JinXJinX]

change "https://github.com/flutter/plugins/tree/master/packages/in_app_purchase/..." to "https://github.com/flutter/plugins/tree/master/packages/in_app_purchase/in_app_purchase/..."

List which issues are fixed by this PR. You must list at least one issue.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

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• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
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[cyanglaz]

@JinXJinX Thanks a lot for the PR. I'm working on a Pr that tries to improve the README of the plugin. #3731

Most of your fixes are addressed in that PR. And I included your fix that wasn't addressed.

I'm going to close this PR as the other PR would land soon and fix the issues here.