Update documentation

Author: grahamearley

Firestore.js

@@ -79,7 +79,7 @@ var Firestore = function (email, key, projectId) {
    * @param {string} path the path of the document to update.
    *                      If document name not provided, a random ID will be generated.
    * @param {object} fields the document's new fields
-   * @param {boolean} if true, the update will use a mask
+   * @param {boolean} mask if true, the update will use a mask
    * @return {object} the Document object written to Firestore
    */
   this.updateDocument = function (path, fields, mask) {