effective_go: move 'Type switch' section into 'Control structures' section.

Needs to be an h3, not an h2.
Thanks to Mingjie Xing for pointing it out.

LGTM=dsymonds
R=golang-codereviews, dsymonds
CC=golang-codereviews
https://golang.org/cl/55980046

doc/effective_go.html

@@ -850,7 +850,7 @@ func Compare(a, b []byte) int {
 }
 </pre>
 
-<h2 id="type_switch">Type switch</h2>
+<h3 id="type_switch">Type switch</h3>
 
 <p>
 A switch can also be used to discover the dynamic type of an interface