Oh, good catch Ioannis.
Might be easy to fix, probably by looking at one of the other plug-ins that correctly display a license.
I'm looking at our testing server, and the Mailer Plug-in displays the MIT License correctly in the about page. By clicking on the plug-in name, and going to the Wiki page, the source code points to https://github.com/jenkinsci/mailer-plugin
The mailer-plugin does not have a LICENSE.txt. So I suspect what we are missing is the <license> tag in pom.xml.
Hopefully that should add the correct license after the next release.