XEP-0045: Improve 'Service Removes Non-Member' example

[guusdk]

The pre-existing example appears to show two mutually exclusive acceptable stanzas. This can be confused for a requirement to send two stanzas. In this commit, the example is improved by removing one of the variants. A small textual change has been applied to define the optionality of the 'actor' element.

This change has been discussed:

• on the standards mailing list.
• in the XSF Discussion chat room.