Docs: Fix typo and remove duplicate word.
Docs: Work on "customising an app".

The docs contained reference on how to keep template tags working after
overriding an app, but this information is now obsolete as they've been
moved to oscar/templatetags. Thus, this info is removed.
It is replaced by similar instructions on how to keep the Django admin
around.
Merge pull request #496 from maikhoepfel/fix/admindocs

Docs: Update 'How to customise an app'