fix spelling

xep-0198.xml

@@ -513,7 +513,7 @@
 </section1>
 
 <section1 topic='Security Considerations' anchor='security'>
-  <p>As noted, a server MUST NOT allow an client to resume a stream management session until after the client has authenticated (for some value of "authentication"); this helps to prevent session hijacking.</p>
+  <p>As noted, a server MUST NOT allow a client to resume a stream management session until after the client has authenticated (for some value of "authentication"); this helps to prevent session hijacking.</p>
 </section1>
 
 <section1 topic='IANA Considerations' anchor='iana'>