fix: upgrade multiple dependencies with Snyk

Snyk has created this PR to upgrade:
  - react from 16.8.6 to 16.12.0.
    See this package in NPM: https://www.npmjs.com/package/react
  - react-dom from 16.8.6 to 16.12.0.
    See this package in NPM: https://www.npmjs.com/package/react-dom

See this project in Snyk:
https://app.snyk.io/org/frantic1048/project/4c1ef258-bda0-4825-919b-b35e2d8b95b4?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

package-lock.json

@@ -23,8 +23,8 @@
         "gatsby-plugin-react-helmet": "3.1.22",
         "gatsby-source-filesystem": "2.1.8",
         "gatsby-transformer-remark": "2.6.10",
-        "react": "16.8.6",
-        "react-dom": "16.8.6",
+        "react": "16.12.0",
+        "react-dom": "16.12.0",
         "react-helmet": "5.2.1",
         "typestyle": "2.0.4"
     },