Update help text

Remove 'space-separated' from help text, only comma works.

https://github.com/jenkinsci/copy-to-slave-plugin/blob/master/src/main/java/com/michelin/cio/hudson/plugins/copytoslave/MyFilePath.java#L106 calls `Util.createFileSet()` which in turn uses a StringTokenizer with only a comma as the delimiter (https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/Util.java#L1083).

This also makes the text consistent with the `CopyToSlaveBuildWrapper` help text: https://github.com/jenkinsci/copy-to-slave-plugin/blob/master/src/main/resources/com/michelin/cio/hudson/plugins/copytoslave/CopyToSlaveBuildWrapper/help-includes.html