Privacy: rename parameter block -> blocked

The field of `item` is already called "blocked" instead of "block" -- it seems that name was changed at some point and this occurrence was forgotten.

Also fixes a double period ("Order of rule.." -> "Order of rule.")

This is intended to be added to #8 and implements exactly the change I proposed in [this comment](https://github.com/strophe/strophejs-plugins/pull/8#issuecomment-29825954).