Change to increment operator in object section

Author: chvmvd

docs/1-trial-session/11-object/_samples/incrementAge/script.js

@@ -1,5 +1,5 @@
 function incrementAge(person) {
-  person.age = person.age + 1;
+  person.age += 1;
   return person;
 }
 

docs/1-trial-session/11-object/index.md

@@ -151,7 +151,7 @@ document.write(nextYearTanaka.age); // 19 と表示されてほしい
 
 ```javascript
 function incrementAge(person) {
-  person.age = person.age + 1;
+  person.age += 1;
   return person;
 }