Tiny change to prevent a non-critical warning when installing on python3

I got an error like this when installing under python 3:

    Getting distribution for 'collective.recipe.sphinxbuilder'.
    warning: no files found matching '*.txt'
    warning: no previously-included files matching '*pyc' found anywhere in distribution
      File "..../collective/recipe/sphinxbuilder/docs/conf.py", line 171
        ('index', 'SphinxBuilder.tex', ur'',
                                          ^
    SyntaxError: invalid syntax

      File "..../collective.recipe.sphinxbuilder-0.8.0-py3.3.egg/collective/recipe/sphinxbuilder/docs/conf.py", line 171
        ('index', 'SphinxBuilder.tex', ur'',
                                          ^
    SyntaxError: invalid syntax

    Got collective.recipe.sphinxbuilder 0.8.0.

It is an error that doesn't break the install, but the fix was easy and straightforward.