doc: fix a typo in contribute.html
doc: fix a typo in contribute.html

A fix for a trivial (yet still confusing for neophytes like me!) typo in
contribute.html.

Change-Id: Ic68673fb2a3855c2b9e8042047087450e8793e6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/279452
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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