Fix syntax coloring on emacs

What does this MR do?

Fix the syntax coloring of lib/gitlab/database/migration_helpers.rb in emacs.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

Browser testing is not applicable, other tests I'm kind of relying on this existing one:

  describe '#concurrent_foreign_key_name' do
    it 'returns the name for a foreign key' do
      name = model.concurrent_foreign_key_name(:this_is_a_very_long_table_name,
                                               :with_a_very_long_column_name)

      expect(name).to be_an_instance_of(String)
      expect(name.length).to eq(13)
    end
  end

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Security

Not applicable