spec: delete spurious tag.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4536082

doc/go_spec.html

@@ -1180,7 +1180,6 @@ maps grow to accommodate the number of items
 stored in them, with the exception of <code>nil</code> maps.
 A <code>nil</code> map is equivalent to an empty map except that no elements
 may be added.
-</code>
 
 <h3 id="Channel_types">Channel types</h3>