XEP-0403: Fix use of MIX-Presence namespace

The examples used "urn:xmpp:presence:0" which is not the correct
namespace according to the text, it should be "urn:xmpp:mix:presence:0".

xep-0403.xml

@@ -200,7 +200,7 @@
 <items node='urn:xmpp:mix:nodes:presence'>
   <item id='123456#coven@mix.shakespeare.example/UUID-x4r/2491'>
     <presence xmlns='jabber:client'>
-      <mix xmlns='urn:xmpp:presence:0'>
+      <mix xmlns='urn:xmpp:mix:presence:0'>
         <jid>hecate@shakespeare.example/UUID-x4r/2491</jid>
         <nick>thirdwitch</jid>
       </mix>
@@ -265,7 +265,7 @@ A user MAY share presence information with the channel, for one or more online c
 <![CDATA[<presence from='123435#coven@mix.shakespeare.example/UUID-a1j/7533'
           to='hag99@shakespeare.example'
           id='77E07BB0-55CF-4BD4-890E-3F7C0E686BBD'>
-  <mix xmlns='urn:xmpp:presence:0'>
+  <mix xmlns='urn:xmpp:mix:presence:0'>
      <jid>hecate@shakespeare.example/UUID-x4r/2491</jid>
      <nick>thirdwitch</jid>
   </mix>
@@ -313,7 +313,7 @@ A user MAY share presence information with the channel, for one or more online c
           to='hecate@shakespeare.example'
           id='77E07BB0-55CF-4BD4-890E-3F7C0E686BBD'
           type='unavailable'>
-      <mix xmlns='urn:xmpp:presence:0'>
+      <mix xmlns='urn:xmpp:mix:presence:0'>
         <jid>hecate@shakespeare.example/UUID-x4r/2491</jid>
         <nick>thirdwitch</jid>
       </mix>
@@ -337,7 +337,7 @@ A user MAY share presence information with the channel, for one or more online c
           to='hecate@shakespeare.example'
           id='77E07BB0-55CF-4BD4-890E-3F7C0E686BBD'
           type='unavailable'>
-      <mix xmlns='urn:xmpp:presence:0'>
+      <mix xmlns='urn:xmpp:mix:presence:0'>
         <jid>hecate@shakespeare.example/UUID-x4r/2491</jid>
         <nick>thirdwitch</jid>
       </mix>