docs(toJson): improve option param documentation

With an upgrade to dgeni-packages 0.10.13, this style of optional param
is rendered more correctly.

See angular#11095

Author: petebacondarwin

src/Angular.js

@@ -979,8 +979,8 @@ function toJsonReplacer(key, value) {
  * stripped since angular uses this notation internally.
  *
  * @param {Object|Array|Date|string|number} obj Input to be serialized into JSON.
- * @param {boolean|number=} pretty If set to true, the JSON output will contain newlines and whitespace.
- *    If set to an integer, the JSON output will contain that many spaces per indentation (the default is 2).
+ * @param {boolean|number} [pretty=2] If set to true, the JSON output will contain newlines and whitespace.
+ *    If set to an integer, the JSON output will contain that many spaces per indentation.
  * @returns {string|undefined} JSON-ified string representing `obj`.
  */
 function toJson(obj, pretty) {