Fix GNU Mailman license

README.md

@@ -326,7 +326,7 @@ _Mailing lists servers and mass mailing software - one message to many recipient
 
 - [Dada Mail](http://dadamailproject.com/) - Web-based list management system that can be used for announcement lists and/or discussion lists. ([Source Code](https://github.com/justingit/dada-mail)) `GPL-2.0` `Perl`
 - [Mail For Good](https://github.com/freeCodeCamp/mail-for-good) `⚠` - Open source email campaign management tool for nonprofits. `BSD` `Javascript`
-- [Mailman](https://www.gnu.org/software/mailman/) - The Gnu mailing list server. `GPL-1.0` `Python`
+- [Mailman](https://www.gnu.org/software/mailman/) - The Gnu mailing list server. `GPL-3.0` `Python`
 - [Mailtrain](https://mailtrain.org/) - self hosted newsletter application built on Node.js (v5+) and MySQL (v5.5+ or MariaDB). ([Source Code](https://github.com/andris9/mailtrain)) `GPL-3.0` `Nodejs`
 - [MailyHerald](http://mailyherald.org/) - Self-hosted Mailchimp alternative that you can easily integrate with your site. Helps you send and manage your application mailings. It support email marketing and conducting the daily stream of notifications you send to your users. ([Source Code](https://github.com/Sology/maily_herald)) `LGPL-3.0` `Ruby`
 - [Mautic](https://www.mautic.org/) - Mautic is marketing automation software (email, social and more). ([Source Code](https://github.com/mautic/mautic)) `GPL-3.0` `PHP`