docs(angular.fromJson): replace the word "Thingy"

Replace the word "thingy" with "JSON string" to specify what thingy means.

Closes #10468

src/Angular.js

@@ -993,7 +993,7 @@ function toJson(obj, pretty) {
  * Deserializes a JSON string.
  *
  * @param {string} json JSON string to deserialize.
- * @returns {Object|Array|string|number} Deserialized thingy.
+ * @returns {Object|Array|string|number} Deserialized JSON string.
  */
 function fromJson(json) {
   return isString(json)