introspection,api: fix Messaging documentation

We don't have a Completed signal.

introspection/org.freedesktop.ModemManager1.Modem.Messaging.xml

@@ -27,9 +27,8 @@
         Retrieve all SMS messages.
 
         This method should only be used once and subsequent information
-        retreived either by listening for the
-        #org.freedesktop.ModemManager1.Modem.Messaging::Added and
-        #org.freedesktop.ModemManager1.Modem.Messaging::Completed signals,
+        retrieved either by listening for the
+        #org.freedesktop.ModemManager1.Modem.Messaging::Added signal,
         or by querying the specific SMS object of interest.
     -->
     <method name="List">
@@ -79,9 +78,7 @@
 
         Check the
         '<link linkend="gdbus-property-org-freedesktop-ModemManager1-Sms.State">State</link>'
-        property to determine if the message is complete. The
-        #org.freedesktop.ModemManager1.Modem.Messaging::Completed
-        signal will also be emitted when the message is complete.
+        property to determine if the message is complete.
     -->
     <signal name="Added">
       <arg name="path" type="o" />