Added a small modifications in the code formatting of background subtraction tutorial

doc/tutorials/video/background_subtraction/background_subtraction.rst

@@ -178,8 +178,9 @@ The results as well as the input data are shown on the screen.
       pMOG2->apply(frame, fgMaskMOG2);
       //get the frame number and write it on the current frame
       size_t index = fn.find_last_of("/");
-      if(index == string::npos)
+      if(index == string::npos) {
         index = fn.find_last_of("\\");
+      }
       size_t index2 = fn.find_last_of(".");
       string prefix = fn.substr(0,index+1);
       string suffix = fn.substr(index2);