Update repository URL of mock plugin

[NotMyFault]

cc @jglick as a maintainer, what do you think? This would match with the other slave -> agent repos.

[jglick]

Would make

<gitHubRepo>jenkinsci/${project.artifactId}-plugin</gitHubRepo>

not be accurate; to avoid confusion I prefer to have the repo name align with the artifactId. The display name and description have already been updated so this seems unnecessary.

[NotMyFault]

No strong objections either way. I just thought it would align better with the other renamed repositories.

[jglick]

…which are kind of a mess, because now we have https://github.com/jenkinsci/ssh-agent-plugin (plugin name ssh-agent) which runs the ssh-agent tool (an actual agent), and https://github.com/jenkinsci/ssh-agents-plugin (plugin name ssh-slaves) which runs slaves agents over SSH.

No strong opinion either, just a general preference to keep repo URLs aligned with artifactIds which must be stable.