Use simpler syntax for admonition

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/client/base/client.py

@@ -103,7 +103,7 @@ async def main():
                     break
         ```
 
-        !!! Note
+        Note:
             * In this case a very simple `GrpcStreamBroadcaster` is used, asuming that
                 each call to `example_stream` will stream the same data. If the request
                 is more complex, you will probably need to have some kind of map from