Modify documentation for switch from DNSServer to AsyncDNSServer

Author: ZongyiYang

README.md

@@ -780,7 +780,7 @@ IPAddress APStaticSN  = IPAddress(255, 255, 255, 0);
 #define HTTP_PORT           80
 
 AsyncWebServer webServer(HTTP_PORT);
-DNSServer dnsServer;
+AsyncDNSServer dnsServer;
 
 ///////////////////////////////////////////
 // New in v1.4.0
@@ -1222,7 +1222,7 @@ void printLocalTime()
 #define HTTP_PORT           80
 
 AsyncWebServer webServer(HTTP_PORT);
-DNSServer dnsServer;
+AsyncDNSServer dnsServer;
 
 ESPAsync_WiFiManager ESPAsync_wifiManager(&webServer, &dnsServer);
 ```

src/ESPAsync_WiFiManager-Impl.h

@@ -346,7 +346,7 @@ void ESPAsync_WiFiManager::setupConfigPortal()
   {
     dnsServer->setErrorReplyCode(AsyncDNSReplyCode::NoError);
 
-    // DNSServer started with "*" domain name, all DNS requests will be passsed to WiFi.softAPIP()
+    // AsyncDNSServer started with "*" domain name, all DNS requests will be passsed to WiFi.softAPIP()
     if (! dnsServer->start(DNS_PORT, "*", WiFi.softAPIP()))
     {
       // No socket available