build: bump ffi from 1.10.0 to 1.11.0 in /docs

Bumps [ffi](https://github.com/ffi/ffi) from 1.10.0 to 1.11.0.
- [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/1.10.0...1.11.0

)

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

docs/Gemfile.lock

@@ -28,7 +28,7 @@ GEM
     execjs (2.7.0)
     faraday (0.15.4)
       multipart-post (>= 1.2, < 3)
-    ffi (1.10.0)
+    ffi (1.11.0)
     forwardable-extended (2.6.0)
     gemoji (3.0.1)
     github-pages (198)