Update en-US.mdx (#36)

Co-authored-by: Yangshun Tay <tay.yang.shun@gmail.com>

Author: aravindkarteekr

questions/explain-how-this-works-in-javascript/en-US.mdx

@@ -105,7 +105,7 @@ const person = new Person('John');
 person.showThis(); // Person {name: 'John'}
 
 const showThisStandalone = person.showThis;
-showThisStandalone(); // `undefined` because all parts of a class' body are strict mode.
+showThisStandalone(); // `undefined` because in JavaScript class bodies, all methods are strict mode by default, even if you don't add 'use strict'
 ```
 
 ### Explicitly binding `this`