Rewrite 'multi' option in an update query to 'multiple'.

Convert 'multi' option to 'multiple'.

PHP's MongoCollection uses 'multiple' (http://php.net/manual/en/mongocollection.update.php)
but MongoDB uses 'multi' (http://docs.mongodb.org/manual/reference/method/db.collection.update)

It's easy to write 'multi' like you would in native queries and other libraries
and not notice that PHP has a different name.