fix readme apply -f link for v0.4.0 release

Apparently `dashboard-latest-release.yaml` was renamed `dashboard_latest_release.yaml` on v0.4.0, here is the fix 😁

README.md

@@ -89,7 +89,7 @@ You can then access the Tekton Dashboard at `tekton-dashboard.${ip}.nip.io`. Thi
 1. Assuming you want to install the Dashboard into the `tekton-pipelines` namespace:
 
    ```bash
-   kubectl apply --filename https://github.com/tektoncd/dashboard/releases/download/v0.4.0/dashboard-latest-openshift-tekton-dashboard-release.yaml --validate=false
+   kubectl apply --filename https://github.com/tektoncd/dashboard/releases/download/v0.4.0/dashboard_latest_openshift-tekton-dashboard-release.yaml --validate=false
    ```
 
 2. Access the dashboard by determining its route with `kubectl get route tekton-dashboard -n tekton-pipelines`