Fix examples; add syntax highlighting

Example code was leading to the error:

    TypeError: Object { _id: 517bf7d9297726e856000001 } has no method 'attach'

See https://github.com/firebaseco/mongoose-attachments-aws2js/blob/master/lib/aws2js-provider.js#L52

Assigning the result of `require('mongoose-attachments')` to `var attachments` was then leading to the error:

    Error: Storage Provider "s3" can not be found

See https://github.com/firebaseco/mongoose-attachments-aws2js/blob/master/lib/aws2js-provider.js#L49

I've changed the `providerName` configuration parameter to `'aws2js'` and now everything is working.