Fix broken link to Apple Developer Program

There's no longer an iOS Developer Program, only one Apple Developer
Program for all Apple Platforms (iOS, macOS, tvOS, watchOS).

Change-Id: Id330ea8958e4d87ec974561821c4bb6d3da26b81
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

doc/src/external-resources.qdoc

@@ -45,6 +45,11 @@
     \title Xcode
 */
 
+/*!
+    \externalpage https://developer.apple.com/programs/
+    \title Apple Developer Program
+*/
+
 /*!
     \externalpage http://help.apple.com/xcode/mac/8.0/#/dev10510b1f7
     \title Create asset catalogs and sets

doc/src/platforms/ios.qdoc

@@ -68,8 +68,7 @@
     For running Qt applications in the simulator that comes with Xcode,
     this is all you need. However, for running applications on a
     device and publishing your applications in the App Store, you must
-    join the iOS Developer Program (see
-    \l{https://developer.apple.com/programs/ios/}), and set up
+    join the \l{Apple Developer Program}, and set up
     developer certificates and provisioning profiles. The easiest
     solution is to use a profile that takes any App ID (a \c *).