docs(webhook-receivers.md): fix broken links

fix https://github.com/fluxcd/website/runs/4156089344

fix 2 errors reported by lychee Link Checker:
Errors in content/en/docs/guides/webhook-receivers.md
✗ file:///github/workspace/content/en/docs/installation (Cannot find file file:///github/workspace/content/en/docs/installation)
✗ file:///github/workspace/content/en/docs/get-started/index.md (Cannot find file file:///github/workspace/content/en/docs/get-started/index.md)

Signed-off-by: lloydchang <lloydchang@gmail.com>

content/en/docs/guides/webhook-receivers.md

@@ -15,8 +15,8 @@ every time a source changes. Using webhook receivers make
 
 To follow this guide you'll need a Kubernetes cluster with the GitOps 
 toolkit controllers installed on it.
-Please see the [get started guide](../get-started/index.md)
-or the [installation guide](../installation/).
+Please see the [get started guide](../get-started.md)
+or the [installation guide](../installation.md).
 
 The [notification controller](../components/notification/_index.md)
 can handle events coming from external systems