[Fix serverlessworkflow#873] Fixing error in export definition

I forgot to include export definition in the task.

Author: fjtirado

schema/workflow.yaml

@@ -110,6 +110,9 @@ $defs:
       output:
         $ref: '#/$defs/output'
         description: Configure the task's output.
+      export:
+        $ref: '#/$defs/export'
+        description: Exports data to content.
       timeout:
         $ref: '#/$defs/timeout'
         description: The task's timeout configuration, if any.