[Fix] Typo integration-doc.md, s/input/output

plasma_framework/docs/integration-docs/integration-doc.md

@@ -255,7 +255,7 @@ When listening for Exit Game related events, it's important to remember that the
         uint16 inputIndex
     );
 ```
-- When an input has been piggybacked on an In-flight Exit.
+- When an output has been piggybacked on an In-flight Exit.
 ```
     event InFlightExitOutputPiggybacked(
         address indexed exitTarget,