net/http: stop ExampleServer_Shutdown from hanging on error

Running the example code when not having permissions
to bind to port 80 will cause the program to hang after
printing the error message.

Change-Id: I2433ba2629b362fc8f1731e40cab5eea72ec354f
GitHub-Last-Rev: 0bb3dc08b6f646470fc6ff208ea12bca901a2299
GitHub-Pull-Request: golang/go#32947
Reviewed-on: https://go-review.googlesource.com/c/go/+/185157Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/example_test.go

@@ -132,7 +132,7 @@ func ExampleServer_Shutdown() {
 
 	if err := srv.ListenAndServe(); err != http.ErrServerClosed {
 		// Error starting or closing listener:
-		log.Printf("HTTP server ListenAndServe: %v", err)
+		log.Fatalf("HTTP server ListenAndServe: %v", err)
 	}
 
 	<-idleConnsClosed