doc: add alt attribute to gopher image in help.html

This adds an alt tag for accessibility. The alt text is a visual
description of the text that is read out loud to users using a
screen reader. The HTML specifications indicate that alt tags for
decorative images should be left blank.

Fixes #39861

Change-Id: I76c39a461ceabe685826aa46e4f26ad893d50634
Reviewed-on: https://go-review.googlesource.com/c/go/+/240258
Reviewed-by: Alexander Nohe <alex.nohe427@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/doc/help.html b/doc/help.html
index b305a57..3d32ae5 100644
--- a/doc/help.html
+++ b/doc/help.html
@@ -8,7 +8,7 @@
 
 <h2 id="help">Get help</h2>
 
-<img class="gopher" src="/doc/gopher/help.png"/>
+<img class="gopher" src="/doc/gopher/help.png" alt=""/>
 
 {{if not $.GoogleCN}}
 <h3 id="mailinglist"><a href="https://groups.google.com/group/golang-nuts">Go Nuts Mailing List</a></h3>