Fix README for accessing ErrorBoundary
Fix README for accessing ErrorBoundary

Not sure if the code in the README works as is (didn't test) but it's at least against typescript definitions and differs from the documented usage: https://docs.bugsnag.com/platforms/javascript/react/
bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js