fix grammar: "only exist" s/b "exist only"

To say features "only exist" in JavaScript means that the features don't do anything else, they only exist.
JS is unique because it's the only browser technology which has its feature set; the features exist only in JS.

Author: bgates

1-js/01-getting-started/1-intro/article.md

@@ -89,7 +89,7 @@ There are at least *three* great things about JavaScript:
 + Supported by all major browsers and enabled by default.
 ```
 
-Combined, these 3 things only exist in JavaScript and no other browser technology.
+Combined, these 3 things exist only in JavaScript and no other browser technology.
 
 That's what makes JavaScript unique. That's why it is the most widespread way of creating browser interfaces.