Doc: Change CheckInput example to valid JSON

JSON does not allow trailing commas.
When using the current example you get:
"Invalid application/json in request"

Change-Id: Iff346918f453ff6bd5c47a0494046b45872e1fb1

resources/Documentation/rest-api-checks.md

diff --git a/resources/Documentation/rest-api-checks.md b/resources/Documentation/rest-api-checks.md
index fd31b87..7308a18 100644
--- a/resources/Documentation/rest-api-checks.md
+++ b/resources/Documentation/rest-api-checks.md
@@ -114,7 +114,7 @@
     "checker_uuid": "test:my-checker",
     "state": "RUNNING",
     "url": "https://foo.corp.com/test-checker/results/123",
-    "started": "2019-01-31 09:59:32.126000000",
+    "started": "2019-01-31 09:59:32.126000000"
   }
 ```