fix name of translation catalog

workspace/discover extracts strings to a catalog named plasma_applet_org.kde.muonnotifier.pot but discover/notifier/plasmoid/metadata.desktop has
X-KDE-PluginInfo-Name=org.kde.discovernotifier
therefore the existing translation catalog plasma_applet_org.kde.muonnotifier.po is not loaded and the plasmoid is untranslated
to fix this error the catalog has to be renamed to plasma_applet_org.kde.discovernotifier
REVIEW:D1824