fix: upgrade prismjs from 1.19.0 to 1.20.0

Snyk has created this PR to upgrade prismjs from 1.19.0 to 1.20.0.

See this package in NPM:
https://www.npmjs.com/package/prismjs

See this project in Snyk:
https://app.snyk.io/org/philwareham/project/2cfa954e-5edf-44e0-8e6d-c4dc2a495761?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

package.json

@@ -50,7 +50,7 @@
     "bootstrap": "3.4.1",
     "jquery": "3.4.1",
     "jquery-ui-dist": "1.12.1",
-    "prismjs": "1.19.0",
+    "prismjs": "1.20.0",
     "scrollspy-js": "0.1.3"
   },
   "optionalDependencies": {