Fix docs for markers
Fix docs for markers

When having a requirement for just a specific python version,
the correct check for i.e. python 2.6 is "python_version=='2.6'"

Change-Id: I65a7481dd0575f105839685ad037d8921d64286a

openstack-dev/pbr,openstack-dev/pbr,klmitch/pbr,openstack-dev/pbr,klmitch/pbr