update samples: waitKey() usage

Original commit is a5f19f7dd6

samples/cpp/3calibration.cpp

@@ -351,7 +351,7 @@ int main( int argc, char** argv )
         for( k = 0; k < small_canvas.rows; k += 16 )
             line(small_canvas, Point(0, k), Point(small_canvas.cols, k), Scalar(0,255,0), 1);
         imshow("rectified", small_canvas);
-        int c = waitKey(0);
+        char c = (char)waitKey(0);
         if( c == 27 || c == 'q' || c == 'Q' )
             break;
     }