Un-hardcode LED pin in AdvancedWebServer example (#909)

Partial #908

Author: earlephilhower

libraries/WebServer/examples/AdvancedWebServer/AdvancedWebServer.ino

@@ -43,7 +43,7 @@ const char *password = STAPSK;
 
 WebServer server(80);
 
-const int led = 13;
+const int led = LED_BUILTIN;
 
 void handleRoot() {
   digitalWrite(led, 1);