Update README.md

fix #76

README.md

@@ -366,7 +366,7 @@ In place of common selectors like class, id or attribute we can use `document.qu
 
     // Native
     function getHeight(el) {
-      const styles = this.getComputedStyles(el);
+      const styles = window.getComputedStyle(el);
       const height = el.offsetHeight;
       const borderTopWidth = parseFloat(styles.borderTopWidth);
       const borderBottomWidth = parseFloat(styles.borderBottomWidth);