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/107378Reviewed-by: Mohit Bajoria <mohitbajo36@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -585,8 +585,8 @@ Fixes #159
 <h3>First line</h3>
 
 <p>
-The first line of the change description is conventionally a one-line
-summary of the change, prefixed by the primary affected package. Keep the first line as short as possible (ideally, under 76 characters).</p>
+The first line of the change description is conventionally a short one-line
+summary of the change, prefixed by the primary affected package.
 
 <p>It should be written so to complete the sentence "This change modifies Go to _____."</p>
 
@@ -596,7 +596,7 @@ summary of the change, prefixed by the primary affected package. Keep the first
 change and explain what it does.
 Write in complete sentences with correct punctuation, just like
 for your comments in Go.
-No Markdown should be used.
+Don't use HTML, Markdown, or any other markup language.
 </p>
 
 <h3>Referencing issues</h3>