improve error position marker slightly

before:
  $ bin/jsonlint bad.json
  bad.json: Parse error on line 2:
  ...th trailing comma",}
  ----------------------^
  Expected: 'STRING' - It appears you have an extra trailing comma

after:
  $ bin/jsonlint bad.json
  bad.json: Parse error on line 2:
  ...th trailing comma",}
  ---------------------^