FCPPluginClient: Fix post-purge-db4o-merge glitch

The class has been moved from freenet.node.fcp to freenet.clients.fcp as
part of merging purge-db4o into plugin-fcp-rewrite.
While I fixed most references to the old package, I forgot one which
this commit fixes: The JavaDoc of FCPPluginClient contains a Logger
configuration instruction which still used the old package name. As that
is not JavaDoc-linkified, it was missed during the merge.