Bump express from 4.13.4 to 4.20.0 in /web/xterm.js

Bumps [express](https://github.com/expressjs/express) from 4.13.4 to 4.20.0.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/master/History.md)
- [Commits](expressjs/express@4.13.4...4.20.0)

---
updated-dependencies:
- dependency-name: express
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

web/xterm.js/package.json

@@ -18,7 +18,7 @@
     "derequire": "^2.0.3",
     "docdash": "0.4.0",
     "exorcist": "^0.4.0",
-    "express": "4.13.4",
+    "express": "4.20.0",
     "express-ws": "2.0.0-rc.1",
     "glob": "^7.0.5",
     "jsdoc": "3.4.0",