Update spec to complete the example section

Author: jcarter3

spec.md

@@ -525,7 +525,7 @@ For example, if the client pushed a manifest with the following tags:
 PUT /v2/<name>/manifests/<digest>?tag=1.2.3&tag=1.2&tag=1&tag=latest 
 ```
 
-The server could respond with the following `OCI-Tag` field line(s) in the response header:
+The server could respond with the following headers:
 ```
 OCI-Tag: 1.2.3, 1.2, 1
 OCI-Tag: latest