Fix README.md#Dynamic Compilation

According to [src/coffeeCoverage.coffee#L336](https://github.com/benbria/coffee-coverage/blob/master/src/coffeeCoverage.coffee#L336) and [src/helpers.coffee#L7](https://github.com/benbria/coffee-coverage/blob/master/src/helpers.coffee#L7), the item in exclude list must not begin with a slash.