Update dependency gatsby to v2.20.8

Author: renovate-bot

docs/package-lock.json

@@ -6,7 +6,7 @@
     "serve": "gatsby serve"
   },
   "dependencies": {
-    "gatsby": "2.20.4",
+    "gatsby": "2.20.8",
     "gatsby-theme-apollo-docs": "4.1.1",
     "react": "16.13.1",
     "react-dom": "16.13.1"