Update docs/how-to/cooperative_groups.rst

Co-authored-by: Leo Paoletti <[email protected]>

docs/how-to/cooperative_groups.rst

@@ -278,7 +278,7 @@ This section describes the necessary step to be able to synchronize group over a
         return 0;
     }
 
-2. Launch the cooperative kernel on single GPU:
+2. Launch the cooperative kernel on a single GPU:
 
 .. code-block:: C++