fix: Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-COMMONMARKER-5829860

Author: snyk-bot

Gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-gem 'github-pages', group: :jekyll_plugins
+gem 'github-pages', '>= 226', group: :jekyll_plugins
 gem 'jekyll'
 gem 'html-proofer', '>= 3.10.0'
 gem 'jekyll-redirect-from'