Sema: Diagnostic strings should not end with a full stop

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -5342,7 +5342,7 @@ ERROR(actor_protocol_illegal_inheritance,none,
       "non-actor type %0 cannot conform to the %1 protocol",
       (DeclName, DeclName))
 ERROR(distributed_actor_conformance_missing_system_type,none,
-      "distributed actor %0 does not declare ActorSystem it can be used with.",
+      "distributed actor %0 does not declare ActorSystem it can be used with",
       (DeclName))
 NOTE(note_distributed_actor_system_can_be_defined_using_defaultdistributedactorsystem,none,
      "you can provide a module-wide default actor system by declaring:\n"