Update iOS link to geo-detect show correct store listing

Adding 'geo.' at beginning of the URL will automatically show users the store listing for their region so it's a better experience.

For example users in Japan will see: https://itunes.apple.com/jp/app/nextcloud/id1125420102?mt=8 which has local customer reviews and certain other information in Japanese.

themes/example/defaults.php

@@ -41,7 +41,7 @@ class OC_Theme {
 	 * @return string URL
 	 */
 	public function getiOSClientUrl() {
-		return 'https://itunes.apple.com/us/app/nextcloud/id1125420102?mt=8';
+		return 'https://geo.itunes.apple.com/us/app/nextcloud/id1125420102?mt=8';
 	}
 
 	/**