Convert indention into spaces instead of tabs

I just saw that the indention in the [target_installer_spec](https://github.com/CocoaPods/CocoaPods/blob/740dfc73c4b866d5d559f2be5f899b87b316edd6/spec/unit/installer/target_installer/pod_target_installer_spec.rb#L153-L234) was using tabs and not spaces sometimes. This converts it into spaces.