[docs] Fix link to rules_apple tutorial
Looks like a search and replace went wrong in This is an external link, so it shouldn't have been changed, since it was a 404 now.
Closes
@@ -3,4 +3,4 @@ Book: /_book.yaml
# Bazel Tutorial: Build an iOS App
-This tutorial has been moved into the [bazelbuild/rules_apple](https://github.com/bazelbuild/rules_apple/blob/master/doc/start/ios-app).md) repository.
+This tutorial has been moved into the [bazelbuild/rules_apple](https://github.com/bazelbuild/rules_apple/blob/master/doc/tutorials/ios-app.md) repository.
