Fixing typo in pdfbuilder's returned dict

[CatGarab]

I ran into this error when generating a PDF through Sphinx:

Running Sphinx v1.5.6

Exception occurred:
  File "/usr/local/lib/python2.7/dist-packages/sphinx/application.py", line 512, in setup_extension
    mod = __import__(extension, None, None, ['setup'])
  File "/usr/local/lib/python2.7/dist-packages/rst2pdf/pdfbuilder.py", line 916
    'parallel_read_safe': True,
                        ^
SyntaxError: invalid syntax

It appears to be due to a typo in line 915. The change in this PR fixed the error for me.

[akrabat]

Thanks!