plugin-material-theme - update path to material-theme

[timbrown5]

Description

I think my original path was wrong (as I am getting 403s when trying to upload - which I am guessing is becase it's uploading to material-theme, when I have permissions to materialtheme). This updates it to the artifactID, not the plugin namespace (which is io/jenkins/plugins/materialtheme).
https://github.com/jenkinsci/material-theme-plugin

Submitter checklist for adding or changing permissions

Always

• Add link to plugin/component Git repository in description above

Reviewer checklist (not for requesters!)

• Check this if newly added person also needs to be given merge permission to the GitHub repo (please @ the people/person with their GitHub username in this issue as well). If needed, it can be done using an IRC Bot command
• Check that the $pluginId Developers team has Admin permissions while granting the access.
• In the case of plugin adoption, ensure that the Jenkins Jira default assignee is either removed or changed to the new maintainer.
• If security contacts are changed (this includes add/remove), ping the security officer (currently @daniel-beck) in this pull request. If an email contact is changed, wait for approval from the security officer.

There are IRC Bot commands for it