fix doc test

Author: renatomello

src/qibo/models/circuit.py

@@ -914,8 +914,7 @@ def summary(self) -> None:
         the all gates sorted in decreasing number of appearance.
 
         Example:
-            .. testcode::
-
+            .. code-block:: python
                 from qibo import Circuit, gates
 
                 circuit = Circuit(3)