docs: menu entry was renamed a while ago

PR: https://twitter.com/BolverBlitz/status/1082267819365056513

source/manual/how-tos/ipv6_tunnelbroker.rst

@@ -71,7 +71,7 @@ Then under **Interfaces->[OPTX]** check **Enable Interface** and change the
 description to e.g. TUNNELBROKER before hitting **Save**.
 
 The newly created interface must now be set as the default IPv6 gateway
-under **System->Gateways->All** by editing the new gateway entry
+under **System->Gateways->Single** by editing the new gateway entry
 TUNNELBROKER_TUNNELV6 and checking **Default Gateway** before saving.
 
 -----------------------------

source/manual/how-tos/multiwan.rst

@@ -50,7 +50,7 @@ Step 1 - Add monitor IPs
 You may skip this step if you already have setup the monitoring IP and both gateways
 are shown as online.
 
-To add a monitoring IP go to **System->Gateways->All** and click on the first pencil
+To add a monitoring IP go to **System->Gateways->Single** and click on the first pencil
 symbol to edit the first gateway.
 
 Now make sure the following is configured:
@@ -155,7 +155,7 @@ Advanced Options
 ----------------
 For each gateway there are several advanced options you can use to change the
 default behavior/thresholds. These option can be changed under
-**System->Gateways->All**, press the pencil icon next to the Gateway you want
+**System->Gateways->Single**, press the pencil icon next to the Gateway you want
 to update.
 
 The current options are:
@@ -200,7 +200,7 @@ load balance. For instance if you have one line of 10 Mbps and one of 20 Mbps th
 set the weight of the first one to 1 and the second one to 2. This way the second
 gateway will get twice as many traffic to handle than the first.
 
-To do so, go to **System->Gateways->All** and press the pencil icon next to the
+To do so, go to **System->Gateways->Single** and press the pencil icon next to the
 Gateway you want to update. The weight is defined under the advanced section.
 
 ------------------------------