Bump chart.js from 2.3.0 to 2.9.4 in /docs

Bumps [chart.js](https://github.com/chartjs/Chart.js) from 2.3.0 to 2.9.4.
- [Release notes](https://github.com/chartjs/Chart.js/releases)
- [Commits](chartjs/Chart.js@v2.3.0...v2.9.4)

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

Author: dependabot[bot]

docs/package-lock.json

@@ -6,7 +6,7 @@
   "dependencies": {},
   "devDependencies": {
     "autoprefixer": "^9.0.2",
-    "chart.js": "~2.3.0",
+    "chart.js": "~2.9.4",
     "expose-loader": "~0.7.5",
     "object-assign": "~4.1.1",
     "postcss-loader": "~2.1.6",