schema: trivial indentation fix

Signed-off-by: NLaine Stump <laine@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/schemas/network.rng

@@ -237,9 +237,9 @@
               <optional>
                 <ref name="bandwidth"/>
               </optional>
-            <optional>
-              <ref name="vlan"/>
-            </optional>
+              <optional>
+                <ref name="vlan"/>
+              </optional>
             </interleave>
           </element>
         </zeroOrMore>