Fixing URI of object property in usage.rst (#35)

While the example code worked, an explicit prefix must be provided for saving to functional syntax

docs/source/usage.rst

@@ -106,7 +106,7 @@ Instead of writing class expressions as nested constructor calls, some expressio
     A = o.clazz(":A")
     B = o.clazz(":B")
     C = o.clazz(":C")
-    r = o.object_property("r")
+    r = o.object_property(":r")
 
     assert A & B == ObjectIntersectionOf([A, B])
     assert A | B == ObjectUnionOf([A, B])