spec: slightly more realistic example for type assertions

For #17428.

Change-Id: Ia902b50cf0c40e3c2167fb573a39d328331c38c7
Reviewed-on: https://go-review.googlesource.com/31449Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of October 18, 2016",
+	"Subtitle": "Version of October 19, 2016",
 	"Path": "/ref/spec"
 }-->
 
@@ -3120,9 +3120,12 @@ var x interface{} = 7  // x has dynamic type int and value 7
 i := x.(int)                   // i has type int and value 7
 
 type I interface { m() }
-var y I
-s := y.(string)        // illegal: string does not implement I (missing method m)
-r := y.(io.Reader)     // r has type io.Reader and y must implement both I and io.Reader
+
+func f(y I) {
+	s := y.(string)        // illegal: string does not implement I (missing method m)
+	r := y.(io.Reader)     // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
+	…
+}
 </pre>
 
 <p>