Corrected multiply defined labels in docs.

[felixxm]

Latexmk: Index file 'django.idx' was written
Latexmk: Log file says output to 'django.pdf'
Latexmk: List of undefined refs and citations:
  Label `releases/1.2:improved-csrf-protection' multiply defined
  Label `releases/1.2:messages-framework' multiply defined
  Label `releases/1.2:relaxed-requirements-for-usernames' multiply defined
  Label `releases/1.2:support-for-multiple-databases' multiply defined
  Label `releases/1.2:user-passes-test-login-required-and-permission-required' multiply defined
  Label `releases/1.2:what-s-new-in-django-1-2' multiply defined
  Label `topics/signals:connecting-to-signals-sent-by-specific-senders' multiply defined
  Label `topics/templates:template-loading' multiply defined
Latexmk: Summary of warnings from last run of (pdf)latex:
  Latex found 8 multiply defined reference(s)

[carltongibson]

Yep, good. Thanks!

[felixxm]

@carltongibson Thanks for checking 👍