add quotes to ttl example to force string context

Annotation values may not be an integer, they must be handled as a
string.  The current example tested on k8s 1.20.13 generates this error
message:

    json: cannot unmarshal number into Go struct field ObjectMeta.metadata.annotations of type string

docs/tutorials/aws.md

@@ -383,7 +383,7 @@ metadata:
   name: nginx
   annotations:
     external-dns.alpha.kubernetes.io/hostname: nginx.external-dns-test.my-org.com
-    external-dns.alpha.kubernetes.io/ttl: 60
+    external-dns.alpha.kubernetes.io/ttl: "60"
 spec:
     ...
 ```