Update README.rst install command

When I run pip install "django-permissions>=0.5.0" on my system (OS X 10.9.2), it pulls in a different python package: django-permissions 1.0.3 by kai.diefenbach@iqpp.de. If I run pip install "django-permission>=0.5.0", I pull in the correct package
Merge pull request #10 from jongyulin/patch-1

Update README.rst install command