Update Router doc to reflect default field change (#70)

docs/operators/router.md

@@ -54,6 +54,5 @@ An entry that does not match any of the routes is dropped and not processed furt
  routes:
  - output: my_json_parser
  expr: '$.format == "json"'
- - output: catchall
- expr: 'true'
+ default: catchall
 ```