Missing .dates in Range examples.

Fixes #1201, thank you, @jasonqiao36!

Author: honzakral

docs/persistence.rst

@@ -97,8 +97,8 @@ in any of the `range fields
     datetime(2018, 11, 17, 9, 30, 0) in rb.dates # True
 
     # you can also get the limits and whether they are inclusive or exclusive:
-    rb.lower # datetime(2018, 11, 17, 9, 0, 0), True
-    rb.upper # datetime(2018, 11, 17, 10, 0, 0), False
+    rb.dates.lower # datetime(2018, 11, 17, 9, 0, 0), True
+    rb.dates.upper # datetime(2018, 11, 17, 10, 0, 0), False
 
     # empty range is unbounded
     Range().lower # None, False