build(deps): bump ffi from 1.13.0 to 1.13.1 in /docs

Bumps [ffi](https://github.com/ffi/ffi) from 1.13.0 to 1.13.1.
- [Release notes](https://github.com/ffi/ffi/releases)
- [Changelog](https://github.com/ffi/ffi/blob/master/CHANGELOG.md)
- [Commits](https://github.com/ffi/ffi/compare/1.13.0...1.13.1

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

docs/Gemfile.lock

@@ -28,7 +28,7 @@ GEM
     execjs (2.7.0)
     faraday (1.0.1)
       multipart-post (>= 1.2, < 3)
-    ffi (1.13.0)
+    ffi (1.13.1)
     forwardable-extended (2.6.0)
     gemoji (3.0.1)
     github-pages (206)