fix: spell correction to existence

Author: zeel

1-js/04-object-basics/01-object/article.md

@@ -322,7 +322,7 @@ alert( *!*key*/!* in user ); // true, takes the name from key and checks for suc
 ```
 
 ````smart header="Using \"in\" for properties that store `undefined`"
-Usually, the strict comparison `"=== undefined"` check the property existance just fine. But there's a special case when it fails, but `"in"` works correctly.
+Usually, the strict comparison `"=== undefined"` check the property existence just fine. But there's a special case when it fails, but `"in"` works correctly.
 
 It's when an object property exists, but stores `undefined`: