Add '2+ interoperable implementations' line.

Co-authored-by: Sarven Capadisli <info@csarven.ca>

ap-maintenance-wg-charter.html

@@ -240,7 +240,7 @@ <h2>Success Criteria</h2>
       In order to update the Recommendation, each substantive change is expected
       to have <a href="https://www.w3.org/Consortium/Process/#implementation-experience">at least
       two independent implementations</a> of every feature defined in the
-      specification, where interoperability can be verified by passing open test suites.
+      specification, where interoperability can be verified by passing open test suites, and two or more implementations interoperating with each other.
     </p>
 
     <p>