doc/contribute.html: adjust wording from previous CL

doc/contribute.html: adjust wording from previous CL

The previous CL, 107197, overclarified the need for short subject
lines. Tweak the wording to be a guideline (keep it short) rather
than a limit (76 characters), which is more the Go way.

Also be strict about avoiding markup language.

Change-Id: I0da1132db8d86052647d96f1caac60289f2209ce
Reviewed-on: https://go-review.googlesource.com/107378
Reviewed-by: Mohit Bajoria <mohitbajo36@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>