Change weakmap examples' order to make it easier to understand (mdn#2…

…8402)

* Update index.md

* fix: change comment to reflect the reason

files/en-us/web/javascript/reference/global_objects/weakmap/index.md

@@ -75,16 +75,16 @@ const o3 = window;
 wm1.set(o1, 37);
 wm1.set(o2, "azerty");
 wm2.set(o1, o2); // a value can be anything, including an object or a function
-wm2.set(o3, undefined);
+wm2.set(o2, undefined);
 wm2.set(wm1, wm2); // keys and values can be any objects. Even WeakMaps!
 
 wm1.get(o2); // "azerty"
-wm2.get(o2); // undefined, because there is no key for o2 on wm2
-wm2.get(o3); // undefined, because that is the set value
+wm2.get(o2); // undefined, because that is the set value
+wm2.get(o3); // undefined, because there is no key for o3 on wm2
 
 wm1.has(o2); // true
-wm2.has(o2); // false
-wm2.has(o3); // true (even if the value itself is 'undefined')
+wm2.has(o2); // true (even if the value itself is 'undefined')
+wm2.has(o3); // false
 
 wm3.set(o1, 37);
 wm3.get(o1); // 37