fix: upgrade multiple dependencies with Snyk

Snyk has created this PR to upgrade:
  - react from 16.9.0 to 16.12.0.
    See this package in NPM: https://www.npmjs.com/package/react
  - react-dom from 16.9.0 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/o330oei/project/17873807-1905-49d1-9bd2-207b42ee4164?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

deps/npm/docs/package-lock.json

@@ -29,8 +29,8 @@
     "gatsby-transformer-remark": "^2.6.22",
     "prismjs": "^1.17.1",
     "prop-types": "^15.7.2",
-    "react": "^16.9.0",
-    "react-dom": "^16.9.0",
+    "react": "^16.12.0",
+    "react-dom": "^16.12.0",
     "react-helmet": "^5.2.1",
     "rebass": "^4.0.5",
     "styled-components": "^4.4.0"