modules/dispatcher: fix small typo in documentation

modules/dispatcher/doc/dispatcher_admin.xml

@@ -641,7 +641,7 @@ modparam("dispatcher", "force_dst", 1)
 			that are generated locally. E.g.: setting code=408 or class=400 will never set
 			a backend down even if it is, because internally the Kamailio transaction layer
 			generates a 408 in the case of no response from the remote server, and this 
-			internal code 408 is accepted as vaild value.
+			internal code 408 is accepted as valid value.
  		</para>
  		<para>
  		<emphasis>