Update import path for 'monitoring other namespaces' guide

Changed in https://github.com/prometheus-operator/kube-prometheus/commit/42a3ac0606818541e66a111693d3ff98c642f7a8 from `kube-prometheus` to `main`

docs/monitoring-other-namespaces.md

@@ -20,7 +20,7 @@ This is done in the variable `prometheus.roleSpecificNamespaces`. You usually se
 
 Example to create the needed `Role` and `RoleBinding` for the Namespace `foo` : 
 ```
-local kp = (import 'kube-prometheus/kube-prometheus.libsonnet') + {
+local kp = (import 'kube-prometheus/main.libsonnet') + {
   _config+:: {
     namespace: 'monitoring',