doc/contribute.html: capitalize the first letter

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://codereview.appspot.com/109080048

Committer: Dave Cheney <dave@cheney.net>
runtime: export _rt0 for android

LGTM=iant, minux
R=golang-codereviews, minux, iant
CC=golang-codereviews
https://codereview.appspot.com/109470043