Fix documentation build warnings in gpu module reference

modules/gpu/doc/video.rst

@@ -653,7 +653,7 @@ gpu::GMG_GPU
 ------------
 .. ocv:class:: gpu::GMG_GPU
 
-Class used for background/foreground segmentation. ::
+  Class used for background/foreground segmentation. ::
 
     class GMG_GPU_GPU
     {
@@ -677,9 +677,9 @@ Class used for background/foreground segmentation. ::
         ...
     };
 
-The class discriminates between foreground and background pixels by building and maintaining a model of the background. Any pixel which does not fit this model is then deemed to be foreground. The class implements algorithm described in [GMG2012]_.
+  The class discriminates between foreground and background pixels by building and maintaining a model of the background. Any pixel which does not fit this model is then deemed to be foreground. The class implements algorithm described in [GMG2012]_.
 
-Here are important members of the class that control the algorithm, which you can set after constructing the class instance:
+  Here are important members of the class that control the algorithm, which you can set after constructing the class instance:
 
     .. ocv:member:: int maxFeatures