Fixed #20119 -- Fixed typo in auth docs

Thanks vinyll for the report.

docs/topics/auth/customizing.txt

@@ -495,7 +495,7 @@ password resets. You must then provide some key implementation details:
         used as the unique identifier. This will usually be a username of
         some kind, but it can also be an email address, or any other unique
         identifier. The field *must* be unique (i.e., have ``unique=True``
-        set in it's definition).
+        set in its definition).
 
         In the following example, the field ``identifier`` is used
         as the identifying field::