fix: Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-REXML-7577227

Gemfile

@@ -3,5 +3,5 @@ gemspec :name => 'uswds-jekyll'
 
 group :jekyll_plugins do
   # Only put the optional plugins here. Required plugins should go in the gemspec.
-  gem 'jekyll-last-modified-at', '>= 1.3.0'
+  gem 'jekyll-last-modified-at', '>= 1.3.2'
 end