Disclaimer: "I ain't no maven expert, or committer for that matter"
But that patch took about 15 minutes. So one without the slashes shouldn't be too difficult.
However, I personally prefer this way, as now you are explicitly declaring that the "resource" should be an empty directory. Without the trailing slash, a situation where there is a typo in eg. a <source>-element value would create an empty directory instead of complaining that the source file resource can't be found.