(#855) Use correct namespace for external Bundler call

lib/pdk/util/changelog_generator.rb

@@ -9,7 +9,7 @@ module ChangelogGenerator
       # Raises if the github_changelog_generator is not available
       def self.github_changelog_generator_available!
         require 'bundler'
-        raise PDK::CLI::ExitWithError, _('Unable to generate the changelog as the github_changelog_generator gem is not installed') unless Bundler.rubygems.find_name('github_changelog_generator').any?
+        raise PDK::CLI::ExitWithError, _('Unable to generate the changelog as the github_changelog_generator gem is not installed') unless ::Bundler.rubygems.find_name('github_changelog_generator').any?
       end
 
       # Runs the Changelog Generator gem (in the module's context) to automatically create a CHANGLELOG.MD file