Update dependency gatsby-theme-apollo-docs to v3.1.6

Author: renovate-bot

docs/package-lock.json

@@ -7,7 +7,7 @@
   },
   "dependencies": {
     "gatsby": "2.18.2",
-    "gatsby-theme-apollo-docs": "3.1.5",
+    "gatsby-theme-apollo-docs": "3.1.6",
     "react": "16.12.0",
     "react-dom": "16.12.0"
   }