doc: fix typo in The Laws of Reflection article

R=golang-dev, gri, r
CC=golang-dev
https://golang.org/cl/5876047

doc/articles/laws_of_reflection.html

@@ -238,7 +238,7 @@ value (from here on we'll elide the boilerplate and focus just on
 the executable code):
 </p>
 
-{{code "/doc/progs/interface2.go" `/var x/` `/STOP/`}}
+{{code "/doc/progs/interface2.go" `/START f9/` `/STOP/`}}
 
 <p>
 prints

doc/progs/interface2.go

@@ -123,3 +123,10 @@ func f8() {
 	fmt.Println("t is now", t)
 	// STOP OMIT
 }
+
+func f9() {
+	// START f9 OMIT
+	var x float64 = 3.4
+	fmt.Println("value:", reflect.ValueOf(x))
+	// STOP OMIT
+}