Update customizing.txt

The origin statement "which could be ... or whatever" **misguides** many newbies like me.

In fact, the ``login`` function in ``contrib.auth`` stores ``user.pk`` in session, then ``get_user`` function in ``contrib.auth`` gets ``user.pk`` in session and then passes it to your custom ``get_user`` as ``user_id``.

Which means, ``user_id`` prarameter in your custom ``get_user`` has to be the primary key of ``User`` object, too.
Updated test failure example.
Merge pull request #1058 from svisser/ticket_20397

Fixed #20397 - Cleaned up issue with quotation marks in documentation
Fixed #20398 - Added language selection code to example in documentation

Thanks ggbaker for the suggestion and Simeon Visser for the patch.
Merge pull request #1053 from makto/patch-1

Fixed #20394 - Clarified argument of get_user in docs
Fixing a minor spelling mistake in the queryset documentation
Merge pull request #1064 from Wilfred/master

Minor spelling fix