docs/k8s: fix subnet router manifests (#12305)

In https://github.com/tailscale/tailscale/pull/11363
I changed the subnet router manifest to run in tun
mode (for performance reasons), but did not
change the security context to give it net_admin,
which is required to for the tailscale socket.

Updates tailscale/tailscale#12083

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

docs/k8s/subnet.yaml

@@ -29,5 +29,6 @@ spec:
     - name: TS_ROUTES
       value: "{{TS_ROUTES}}"
     securityContext:
-      runAsUser: 1000
-      runAsGroup: 1000
+      capabilities:
+        add:
+        - NET_ADMIN