presence: small spelling fix (bellow -> below)

src/modules/presence/doc/presence_devel.xml

@@ -149,7 +149,7 @@ typedef int (get_rules_doc_t)(str* user, str* domain, str** rules_doc);
 		used in obtaining the status of the subscription and processing the
 		notified body. A reference to the document should be put in the 
 		auth_rules_doc of the subs_t structure given as a parameter to the
-		functions described bellow.
+		functions described below.
 		</para>
 </section>