Bump prismjs from 1.15.0 to 1.21.0

Bumps [prismjs](https://github.com/PrismJS/prism) from 1.15.0 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@v1.15.0...v1.21.0)

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

package.json

@@ -24,7 +24,7 @@
     "gatsby-transformer-remark": "^2.1.6",
     "gatsby-transformer-sharp": "^2.1.3",
     "lodash": "^4.17.19",
-    "prismjs": "^1.15.0",
+    "prismjs": "^1.21.0",
     "react": "^16.5.1",
     "react-dom": "^16.5.1",
     "react-helmet": "^5.2.0",