* Specifies fields to be updated as an expression.
*
* Expression fields are cases such as counter = counter + 1. This method
- * takes precedence over MergeQuery::updateFields() and it's wrappers,
+ * takes precedence over MergeQuery::updateFields() and its wrappers,
* MergeQuery::key() and MergeQuery::fields().
*
* @param $field