docs: add missing fn name for docs build

Author: vkarpov15

lib/document.js

@@ -487,10 +487,11 @@ Document.prototype.update = function update() {
  * @param {Any} val the value to set
  * @param {Schema|String|Number|Buffer|*} [type] optionally specify a type for "on-the-fly" attributes
  * @param {Object} [options] optionally specify options that modify the behavior of the set
+ * @
  * @api public
  */
 
-Document.prototype.$set = function(path, val, type, options) {
+Document.prototype.$set = function $set(path, val, type, options) {
   if (type && utils.getFunctionName(type.constructor) === 'Object') {
     options = type;
     type = undefined;