Update docs.yml (#371)

https://docs.vapi.ai/phone-calling/voicemail-detection takes users to 404. correct redirect to: https://docs.vapi.ai/calls/voicemail-detection. also fixed previous broken redirect link

Author: jimmyechan

fern/docs.yml

@@ -679,4 +679,6 @@ redirects:
   - source: /GHL
     destination: /tools/GHL
   - source: /phone-calling/voice-mail-detection
-    destination: /phone-calling/voicemail-detection
+    destination: /calls/voicemail-detection
+  - source: /phone-calling/voicemail-detection
+    destination: /calls/voicemail-detection