[guide] fix dead es-discourse link

Fixes #2274.

Author: ljharb

README.md

@@ -1859,7 +1859,7 @@ Other Style Guides
 ## Hoisting
 
   <a name="hoisting--about"></a><a name="14.1"></a>
-  - [14.1](#hoisting--about) `var` declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. `const` and `let` declarations are blessed with a new concept called [Temporal Dead Zones (TDZ)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#Temporal_dead_zone). It’s important to know why [typeof is no longer safe](http://es-discourse.com/t/why-typeof-is-no-longer-safe/15).
+  - [14.1](#hoisting--about) `var` declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. `const` and `let` declarations are blessed with a new concept called [Temporal Dead Zones (TDZ)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#Temporal_dead_zone). It’s important to know why [typeof is no longer safe](https://web.archive.org/web/20200121061528/http://es-discourse.com/t/why-typeof-is-no-longer-safe/15).
 
     ```javascript
     // we know this wouldn’t work (assuming there