Fix permission names in path.module help text

For whatever reason, someone made the permission names friendlier in the path.module help text, so it says:

The two permissions are related to URL aliasing are "administer a list of URL aliases" and "add url aliases".

Permission names are however unchanged: "administer url aliases" and "create url aliases", so the help text is misleading. Fix this by telling the users the right permission names, and also fix the multiple "are" in the sentence, so it becomes:

The two permissions related to URL aliasing are "administer url aliases" and "create url aliases".

Simple patch included.