fix typo in runner.js

[tchia04]

BEFORE

$ browserstack-runner --help | grep -i '\-\-browsers'
  --browsers, -b  list of space separatedbrowsers keys as described in json file

AFTER

$ browserstack-runner --help | grep -i '\-\-browsers'
  --browsers, -b  list of space separated browsers keys as described in json file

[tchia04]

It looks like there is some issue with existing test. Even without my change, the travis build is failing