docs(backgroundmode): fix usage markdown
docs(backgroundmode): fix usage markdown

Added missing grave accents.
driftyco/ionic-native,ramonornela/ionic-native,xueron/ionic-native,klirix/ionic-native,driftyco/ionic-native,klirix/ionic-native,xueron/ionic-native,xueron/ionic-native,klirix/ionic-native,driftyco/ionic-native,ramonornela/ionic-native,xueron/ionic-native,ramonornela/ionic-native,ramonornela/ionic-native