change np.ones in Graphene example to sx

docs/source/tutorial/graphene_example.md

@@ -72,10 +72,10 @@ All we need to do is to convert this object to a tight-binding dictionary using
 
 ```{code-cell} ipython3
 def onsite_int(site, U):
-    return U * np.ones((2, 2))
+    return U * sx
 
 def nn_int(site1, site2, V):
-    return V * np.ones((2, 2))
+    return V * sx
 
 builder_int = utils.build_interacting_syst(
     builder=bulk_graphene,