Bump puma from 3.11.2 to 6.3.1 in /docs-book

Bumps [puma](https://github.com/puma/puma) from 3.11.2 to 6.3.1.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.11.2...v6.3.1)

---
updated-dependencies:
- dependency-name: puma
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

docs-book/Gemfile.lock

@@ -156,6 +156,7 @@ GEM
     minitest (5.11.2)
     multi_json (1.13.1)
     multipart-post (2.0.0)
+    nio4r (2.5.9)
     nokogiri (1.8.1)
       mini_portile2 (~> 2.3.0)
     padrino-helpers (0.13.3.4)
@@ -166,7 +167,8 @@ GEM
       activesupport (>= 3.1)
     parallel (1.12.1)
     public_suffix (3.0.1)
-    puma (3.11.2)
+    puma (6.3.1)
+      nio4r (~> 2.0)
     rack (1.6.8)
     rack-livereload (0.3.16)
       rack