Fix for Android phones

Android phones have a operatin system feature that will display a captive portal shortly after connection to a wifi that requires a log in. This is only triggered if you dont use an adress in the private range!

This is a new request based on
 esp8266#5529
and espressif/arduino-esp32#1037

it seems that Android phones will not display the autoredict page if it is in the private IP Range.

I found this fix in the two issues above and tried it with several android phones, non of them displayed it bevore:
Xiaomi Redmi Note 7 (miui 10.3 Android 7) 
Samsung Galaxy A7  (Android 8.0.0) 
Amazon Kindle Fire 7 (Fire OS 5.6.4.0 Android 5.1)

I think for easy access to newcomers this should be changed so it works instantly!

Author: MisterD66

libraries/DNSServer/examples/CaptivePortal/CaptivePortal.ino

@@ -3,7 +3,7 @@
 #include <ESP8266WebServer.h>
 
 const byte DNS_PORT = 53;
-IPAddress apIP(192, 168, 1, 1);
+IPAddress apIP(172, 217, 28, 1); //Use an IP-Adress in the 172.217 Range to enable Popup on Android devices
 DNSServer dnsServer;
 ESP8266WebServer webServer(80);