doc: fix memory model read visibility bug

Fixes #2277.

R=dvyukov, r
CC=golang-dev
https://golang.org/cl/5083044

doc/go_mem.html

@@ -54,7 +54,7 @@ if both of the following hold:
 </p>
 
 <ol>
-<li><span class="event">w</span> happens before <span class="event">r</span>.</li>
+<li><span class="event">r</span> does not happen before <span class="event">w</span>.</li>
 <li>There is no other write <span class="event">w'</span> to <code>v</code> that happens
     after <span class="event">w</span> but before <span class="event">r</span>.</li>
 </ol>