Bug#736119: indentation in python-apt docs

[wh <microrffr@gmail.com>]

Package: python-apt

Version: 0.9.1

in doc/source/library/apt_pkg.rst, lines 718, 726, 736 have different indentation from the other attributes.

rendition:
http://apt.alioth.debian.org/python-apt-doc/library/apt_pkg.html#apt_pkg.Package