fix jsonnet error with kube-prometheus > v0.8

kubelet ServiceMonitor moved to kubernetesControlPlane starting from v0.8

Signed-off-by: elvis chan <78713260+elvischan-crypto@users.noreply.github.com>

jsonnet/lens/custom-prometheus.jsonnet

@@ -24,7 +24,7 @@
       },
     },
   },
-  prometheus+:: {
+  kubernetesControlPlane+:: {
     serviceMonitorKubelet+: {
       spec+: {
         endpoints: std.map(function(endpoint)