fix(jsdoc): mark db.collection callback as optional + typo fix

[nikitaeverywhere]

Hello!

As stated in JSDoc to Db.prototype.collection function,

If the application does not use strict mode you can use it without a
callback in the following way: `var collection = db.collection('mycollection');`

This pull request fixes JSDoc, which removes this IDE warning:

Additionally, it fixes a typo in the function description (duplicated word can).

Thank you for a great project and keep up the good work! 😃