doc/contribute: fix typo and reword a few sentences
doc/contribute: fix typo and reword a few sentences

This change fixes a typo in doc/contribute.html (afects -> affects)
and rewords a few slightly akward sentences.

Change-Id: I6bfbacba0de29464fce134b0fdaf3898a97b8d57
Reviewed-on: https://go-review.googlesource.com/120105
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

josharian/go,golang/go,josharian/go,josharian/go,akutz/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,josharian/go,cloudflare/go,cloudflare/go,akutz/go,golang/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,josharian/go,akutz/go,akutz/go,akutz/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,akutz/go,nawawi/go,josharian/go,CAFxX/go,josharian/go,CAFxX/go,cloudflare/go,josharian/go,cloudflare/go,golang/go