doc: add missing require to example in http2.md

Backport-PR-URL: #22850
PR-URL: #21858
Reviewed-By: Gus Caplan <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Daijiro Wachi <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>

Author: kevinsimper

doc/api/http2.md

@@ -1921,6 +1921,7 @@ instances.
 
 ```js
 const http2 = require('http2');
+const fs = require('fs');
 
 const options = {
   key: fs.readFileSync('server-key.pem'),