Update docs on custom vnet creation

Author: Cecile Robert-Michon

docs/book/src/topics/custom-vnet.md

@@ -17,10 +17,16 @@ spec:
       resourceGroup: custom-vnet
       name: my-vnet
     subnets:
-      - name: control-plane-subnet
+      - name: my-control-plane-subnet
         role: control-plane
-      - name: node-subnet
+        securityGroup:
+          name: my-control-plane-nsg
+      - name: my-node-subnet
         role: node
+        routeTable:
+          name: my-node-routetable
+        securityGroup:
+          name: my-node-nsg
   resourceGroup: cluster-byo-vnet
   ```
 

test/e2e/azure_test.go

@@ -142,7 +142,7 @@ var _ = Describe("Workload cluster creation", func() {
 
 	if os.Getenv("LOCAL_ONLY") != "true" {
 		Context("Creating a private cluster", func() {
-			It("Creates a public management cluster a custom vnet", func() {
+			It("Creates a public management cluster in a custom vnet", func() {
 				clusterName = getClusterName(clusterNamePrefix, "public-custom-vnet")
 				Context("Creating a custom virtual network", func() {
 					Expect(os.Setenv(AzureCustomVNetName, "custom-vnet")).NotTo(HaveOccurred())