fix(jsdoc): mark db.collection callback as optional + typo fix (#1658)

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

* fix(jsdoc): typo

lib/db.js

@@ -393,7 +393,7 @@ var collectionKeys = [
 ];
 
 /**
- * Fetch a specific collection (containing the actual collection information). If the application does not use strict mode you can
+ * Fetch a specific collection (containing the actual collection information). If the application does not use strict mode you
  * can use it without a callback in the following way: `var collection = db.collection('mycollection');`
  *
  * @method
@@ -409,7 +409,7 @@ var collectionKeys = [
  * @param {boolean} [options.strict=false] Returns an error if the collection does not exist
  * @param {object} [options.readConcern=null] Specify a read concern for the collection. (only MongoDB 3.2 or higher supported)
  * @param {object} [options.readConcern.level='local'] Specify a read concern level for the collection operations, one of [local|majority]. (only MongoDB 3.2 or higher supported)
- * @param {Db~collectionResultCallback} callback The collection result callback
+ * @param {Db~collectionResultCallback} [callback] The collection result callback
  * @return {Collection} return the new Collection instance if not in strict mode
  */
 Db.prototype.collection = function(name, options, callback) {