Add unitless example for start_in

doc/ci/yaml/README.md

@@ -1247,6 +1247,7 @@ This is useful if you want to avoid jobs entering `pending` state immediately.
 You can set the period with `start_in` key. The value of `start_in` key is an elapsed time in seconds, unless a unit is
 provided. `start_in` key must be less than or equal to one week. Examples of valid values include:
 
+- `'5'`
 - `10 seconds`
 - `30 minutes`
 - `1 day`