Fix MFA requirement metadata

Author: wkirby

phi_attrs.gemspec

@@ -43,5 +43,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rubocop-rails'
   spec.add_development_dependency 'simplecov', '~> 0.16'
   spec.add_development_dependency 'tzinfo-data'
-  spec.metadata['rubygems_mfa_required'] = false
+  spec.metadata['rubygems_mfa_required'] = 'false'
 end