fix 4.x links

doc/tutorials/introduction/android_binary_package/O4A_SDK.markdown

@@ -82,7 +82,7 @@ The structure of package contents looks as follows:
 
 -   `doc` folder contains various OpenCV documentation in PDF format. It's also available online at
     <http://docs.opencv.org>.
-    @note The most recent docs (nightly build) are at <http://docs.opencv.org/master>. Generally, it's more
+    @note The most recent docs (nightly build) are at <http://docs.opencv.org/4.x>. Generally, it's more
     up-to-date, but can refer to not-yet-released functionality.
     @todo I'm not sure that this is the best place to talk about OpenCV Manager
 

doc/tutorials/introduction/android_binary_package/android_ocl_intro.markdown

@@ -28,7 +28,7 @@ If you need help with anything of the above, you may refer to our @ref tutorial_
 This tutorial also assumes you have an Android operated device with OpenCL enabled.
 
 The related source code is located within OpenCV samples at
-[opencv/samples/android/tutorial-4-opencl](https://github.com/opencv/opencv/tree/master/samples/android/tutorial-4-opencl/) directory.
+[opencv/samples/android/tutorial-4-opencl](https://github.com/opencv/opencv/tree/4.x/samples/android/tutorial-4-opencl/) directory.
 
 Preface
 -------
@@ -255,7 +255,7 @@ As you can see, inheritors for `Camera` and `Camera2` APIs should implement the
 @endcode
 
 Let's leave the details of their implementation beyond of this tutorial, please refer the
-[source code](https://github.com/opencv/opencv/tree/master/samples/android/tutorial-4-opencl/) to see them.
+[source code](https://github.com/opencv/opencv/tree/4.x/samples/android/tutorial-4-opencl/) to see them.
 
 Preview Frames modification
 ---------------------------