Update AddingImages.cpp

If-condition was always true (alpha = 0.5 is set in Line 19).
Now the user input is checked to be between 0 and 1.
This is correct in the tutorial code for OpenCV 2.4.

samples/cpp/tutorial_code/core/AddingImages/AddingImages.cpp

@@ -27,7 +27,7 @@ int main( void )
    std::cin>>input;
 
    // We use the alpha provided by the user if it is between 0 and 1
-   if( alpha >= 0 && alpha <= 1 )
+   if( input >= 0 && input <= 1 )
      { alpha = input; }
 
    //![load]