(maint) Fixes pinned gem in .sync.yml

In 1e5ad00, I had intended to pin github_changelog_generator to
1.16.4, but mistakenly added that pin to the wrong line in .sync.yml.

This commit corrects that mistake and pins github_changelog_generator
to 1.16.4 in .sync.yml.

Author: mhashizume

.sync.yml

@@ -27,8 +27,8 @@ Gemfile:
         from_env: BEAKER_PUPPET_VERSION
         version: '~> 1.22'
       - gem: github_changelog_generator
-      - gem: beaker-module_install_helper
         version: '= 1.16.4'
+      - gem: beaker-module_install_helper
       - gem: concurrent-ruby
         version: '= 1.1.10'
       - gem: beaker-puppet_install_helper