Bump highlight.js from 9.17.1 to 10.4.1 in /book/6-end

Bumps [highlight.js](https://github.com/highlightjs/highlight.js) from 9.17.1 to 10.4.1.
- [Release notes](https://github.com/highlightjs/highlight.js/releases)
- [Changelog](https://github.com/highlightjs/highlight.js/blob/master/CHANGES.md)
- [Commits](highlightjs/highlight.js@9.17.1...10.4.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

book/6-end/package.json

@@ -25,7 +25,7 @@
     "front-matter": "3.0.2",
     "handlebars": "4.5.3",
     "he": "1.2.0",
-    "highlight.js": "9.17.1",
+    "highlight.js": "10.4.1",
     "isomorphic-unfetch": "3.0.0",
     "lodash": "4.17.15",
     "marked": "0.8.0",