examples: Fix YAML error backendRefs in HTTP Header Modifier

The use case "HTTP Header Modifier" has wrong 'backendRefs' service name:
  $kubectl describe HTTPRoute header-http-echo
      Message:               Service "echo" not found
      Observed Generation:   1
      Reason:                BackendNotFound
      Status:                False

The right service name should be "echo-1", which has port number 8080:
  $kubectl describe HTTPRoute header-http-echo
      Message:               Service reference is valid
      Observed Generation:   1
      Reason:                ResolvedRefs
      Status:                True
      Type:                  ResolvedRefs

Fixes: bb50725 ("Header Modifier and Splitting use cases")
Signed-off-by: Haiyue Wang <[email protected]>

examples/kubernetes/gateway/request-header.yaml

@@ -32,5 +32,5 @@ spec:
               - name: my-header-name
                 value: my-header-value
       backendRefs:
-        - name: echo
+        - name: echo-1
           port: 8080