set expiration through API: fix example (quay#1255)

The example looks like a copy/paste from another place and was not updated.
Fix the payload to match was expected.

Author: jbtrystram

modules/setting-tag-expiration-api.adoc

@@ -19,7 +19,7 @@ $ curl -X PUT \
   -H "Authorization: Bearer <bearer_token>" \
   -H "Content-Type: application/json" \
   --data '{
-    "manifest_digest": "<manifest_digest>"
+    "expiration": "<seconds since epoch>"
   }' \
   https://<quay-server.example.com>/api/v1/repository/<namespace>/<repository_name>/tag/<tag>
 ----
@@ -29,4 +29,4 @@ $ curl -X PUT \
 [source,terminal]
 ----
 "Updated"
-----
+----