Update output timestamp SHIP

Add missing `BuildTimestamp` in example.

ships/0037-build-output-timestamp.md

@@ -73,7 +73,7 @@ We will extend the Build resource by an additional (Unix epoch) timestamp entry
 ```yaml
 spec:
   output:
-    timestamp: Zero | SourceTimestamp | <timestamp> | (null)
+    timestamp: Zero | SourceTimestamp | BuildTimestamp | <timestamp> | (null)
 ```
 
 The `timestamp` entry is an optional string value field with the default being `null`. It can have multiple values that lead to different behavior: