Annotation key/value pairs must be strings

As-is, following the docs results in the following error:

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

Signed-off-by: Michael Shen <mishen@umich.edu>

docs/tutorials/aws-sd.md

@@ -221,7 +221,7 @@ 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:
     ...
 ```