documentation: avoid links to 'master' branch from 3.4 maintenance branch

doc/tutorials/ml/introduction_to_pca/introduction_to_pca.markdown

@@ -92,10 +92,10 @@ Source Code
 -----------
 
 This tutorial code's is shown lines below. You can also download it from
-    [here](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/ml/introduction_to_pca/introduction_to_pca.cpp).
+    [here](https://github.com/opencv/opencv/tree/3.4/samples/cpp/tutorial_code/ml/introduction_to_pca/introduction_to_pca.cpp).
 @include cpp/tutorial_code/ml/introduction_to_pca/introduction_to_pca.cpp
 
-@note Another example using PCA for dimensionality reduction while maintaining an amount of variance can be found at [opencv_source_code/samples/cpp/pca.cpp](https://github.com/opencv/opencv/tree/master/samples/cpp/pca.cpp)
+@note Another example using PCA for dimensionality reduction while maintaining an amount of variance can be found at [opencv_source_code/samples/cpp/pca.cpp](https://github.com/opencv/opencv/tree/3.4/samples/cpp/pca.cpp)
 
 Explanation
 -----------

doc/tutorials/ml/non_linear_svms/non_linear_svms.markdown

@@ -87,7 +87,7 @@ Source Code
 -----------
 
 You may also find the source code in `samples/cpp/tutorial_code/ml/non_linear_svms` folder of the OpenCV source library or
-[download it from here](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/ml/non_linear_svms/non_linear_svms.cpp).
+[download it from here](https://github.com/opencv/opencv/tree/3.4/samples/cpp/tutorial_code/ml/non_linear_svms/non_linear_svms.cpp).
 
 @note The following code has been implemented with OpenCV 3.0 classes and functions. An equivalent version of the code
 using OpenCV 2.4 can be found in [this page.](http://docs.opencv.org/2.4/doc/tutorials/ml/non_linear_svms/non_linear_svms.html#nonlinearsvms)