Updated docs example to clear cached_property without raising AttributeError.

Signed-off-by: SaJH <wogur981208@gmail.com>

docs/ref/utils.txt

@@ -521,7 +521,7 @@ https://web.archive.org/web/20110718035220/http://diveintomark.org/archives/2004
     The cached value can be treated like an ordinary attribute of the instance::
 
         # clear it, requiring re-computation next time it's called
-        del person.friends  # or delattr(person, "friends")
+        person.__dict__.pop("friends", None)
 
         # set a value manually, that will persist on the instance until cleared
         person.friends = ["Huckleberry Finn", "Tom Sawyer"]