gnt-cluster(8) fix --backend-parameters opt name

It was mistakenly called --backend

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

man/gnt-cluster.sgml

@@ -373,7 +373,7 @@
       </para>
 
       <para>
-        The <option>--backend</option> option allows you to set
+        The <option>--backend-parameters</option> option allows you to set
         the default backend parameters for the cluster. The parameter
         format is a comma-separated list of key=value pairs with the
         following supported keys: