fix: upgrade gatsby from 5.13.5 to 5.13.6

Snyk has created this PR to upgrade gatsby from 5.13.5 to 5.13.6.

See this package in npm:
gatsby

See this project in Snyk:
https://app.snyk.io/org/mastermindzh/project/6eb54845-2bb5-45e0-80e2-6b3a1c53eee6?utm_source=github&utm_medium=referral&page=upgrade-pr

package.json

@@ -54,7 +54,7 @@
     "@fortawesome/react-fontawesome": "^0.2.2",
     "classnames": "^2.5.1",
     "disqus-react": "^1.1.5",
-    "gatsby": "^5.13.5",
+    "gatsby": "^5.13.6",
     "gatsby-link": "^5.12.1",
     "gatsby-plugin-catch-links": "^5.13.1",
     "gatsby-plugin-feed": "^5.13.1",