doc: correct a minor go_mem.html mistake

Fixes #6227

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13183043

doc/go_mem.html

@@ -361,7 +361,7 @@ func twoprint() {
 
 <p>
 calling <code>twoprint</code> causes <code>"hello, world"</code> to be printed twice.
-The first call to <code>twoprint</code> runs <code>setup</code> once.
+The first call to <code>doprint</code> runs <code>setup</code> once.
 </p>
 
 <h2>Incorrect synchronization</h2>