Fix broken MDN E4X links (#4316)

Author: kidonng

beta/src/pages/blog/2017/11/28/react-v16.2.0-fragment-support.md

@@ -127,7 +127,7 @@ render() {
 
 In React, this desugars to a `<React.Fragment/>` element, as in the example from the previous section. (Non-React frameworks that use JSX may compile to something different.)
 
-Fragment syntax in JSX was inspired by prior art such as the `XMLList() <></>` constructor in [E4X](https://developer.mozilla.org/en-US/docs/Archive/Web/E4X/E4X_for_templating). Using a pair of empty tags is meant to represent the idea it won't add an actual element to the DOM.
+Fragment syntax in JSX was inspired by prior art such as the `XMLList() <></>` constructor in [E4X](https://web.archive.org/web/20201019115828/https://developer.mozilla.org/en-US/docs/Archive/Web/E4X/E4X_for_templating). Using a pair of empty tags is meant to represent the idea it won't add an actual element to the DOM.
 
 ### Keyed Fragments {/*keyed-fragments*/}
 

content/blog/2017-11-28-react-v16.2.0-fragment-support.md

@@ -127,7 +127,7 @@ render() {
 
 In React, this desugars to a `<React.Fragment/>` element, as in the example from the previous section. (Non-React frameworks that use JSX may compile to something different.)
 
-Fragment syntax in JSX was inspired by prior art such as the `XMLList() <></>` constructor in [E4X](https://developer.mozilla.org/en-US/docs/Archive/Web/E4X/E4X_for_templating). Using a pair of empty tags is meant to represent the idea it won't add an actual element to the DOM.
+Fragment syntax in JSX was inspired by prior art such as the `XMLList() <></>` constructor in [E4X](https://web.archive.org/web/20201019115828/https://developer.mozilla.org/en-US/docs/Archive/Web/E4X/E4X_for_templating). Using a pair of empty tags is meant to represent the idea it won't add an actual element to the DOM.
 
 ### Keyed Fragments {#keyed-fragments}