doc: fix typos and grammatical errors in contribute.html
doc: fix typos and grammatical errors in contribute.html

Fixed some typos and grammatical errors in contribute.html

Change-Id: Ifb31f22d876b7bea84b5e130870c26813b2fa139
GitHub-Last-Rev: 43519198859ca8f5e1c870b8f3eeb1b465aa6bc5
GitHub-Pull-Request: golang/go#39892
Reviewed-on: https://go-review.googlesource.com/c/go/+/240277
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,golang/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go