Fix ios doc url (#11262)

dev/tools/dartdoc.dart

diff --git a/dev/tools/dartdoc.dart b/dev/tools/dartdoc.dart
index 43947eb..e50d449 100644
--- a/dev/tools/dartdoc.dart
+++ b/dev/tools/dartdoc.dart
@@ -202,7 +202,7 @@
   );
   indexContents = indexContents.replaceAll(
       'href="iOS/iOS-library.html"',
-      'href="https://docs.flutter.io/objc/"'
+      'href="https://docs.flutter.io/objcdoc/"'
   );
 
   indexFile.writeAsStringSync(indexContents);