[etc] Fix built-in name.

[kijeong]

Changes proposed in this pull request:

• Fix the built-in name "dict" to "dict_" in PIL/PdfImagePlugin.py
  It can be seen an error when: python -c "from Cython.Build import cythonize; cythonize(['PIL/PdfImagePlugin.py'])"

[hugovk]

I agree builtin names should be avoided, and perhaps dictionary is better than dict_.

(There's a chance someone may already be calling this and naming dict as the parameter, but they shouldn't be, as _obj is an internal.)

[kijeong]

I agree that 'dictionary' as name is better for PDF dictionary objects parameter.