Bump rdoc from 3.12.1 to 6.3.1

Bumps [rdoc](https://github.com/ruby/rdoc) from 3.12.1 to 6.3.1.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v3.12.1...v6.3.1)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: direct:production
...

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

Author: dependabot[bot]

Gemfile

@@ -3,7 +3,7 @@ source "https://rubygems.org"
 gem "kramdown"
 gem "nokogiri", '>= 1.5.9'
 gem "jekyll", '>= 1.1.0'
-gem "rdoc", '~> 3.12.1'
+gem "rdoc", '~> 6.3.1'
 gem "haml"
 gem "thor"
 gem 'bourbon'

Gemfile.lock

@@ -27,7 +27,6 @@ GEM
       sass (~> 3.4)
     jekyll-watch (1.5.0)
       listen (~> 3.0, < 3.1)
-    json (1.8.6)
     kramdown (1.13.2)
     liquid (3.0.6)
     listen (3.0.8)
@@ -47,8 +46,7 @@ GEM
     rb-fsevent (0.9.8)
     rb-inotify (0.9.8)
       ffi (>= 0.5.0)
-    rdoc (3.12.1)
-      json (~> 1.4)
+    rdoc (6.3.1)
     rouge (1.11.1)
     safe_yaml (1.0.4)
     sass (3.4.23)
@@ -67,7 +65,7 @@ DEPENDENCIES
   neat
   nokogiri (>= 1.5.9)
   rake
-  rdoc (~> 3.12.1)
+  rdoc (~> 6.3.1)
   sass
   thor