Bump prismjs from 1.19.0 to 1.21.0

Bumps [prismjs](https://github.com/PrismJS/prism) from 1.19.0 to 1.21.0.
- [Release notes](https://github.com/PrismJS/prism/releases)
- [Changelog](https://github.com/PrismJS/prism/blob/master/CHANGELOG.md)
- [Commits](PrismJS/prism@v1.19.0...v1.21.0)

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

package.json

@@ -32,7 +32,7 @@
     "loading-cli": "1.1.0",
     "marked": "1.1.1",
     "path": "0.12.7",
-    "prismjs": "1.19.0",
+    "prismjs": "1.21.0",
     "sitemap-generator": "8.4.2",
     "sqlite3": "4.1.1",
     "stylus": "0.54.7",