Gallery2: Fix spelling error

    <!--  Exit Dialog -->

    <!--  String displayed when exiting with unsaved changes [CHAR LIMIT=NONE] -->

    <string name="unsaved">Discrad the changes you\'ve made to this image?</string>

    <string name="unsaved">Discard the changes you\'ve made to this image?</string>

    <!--  String displayed when exiting with unsaved changes [CHAR LIMIT=NONE] -->

    <string name="save_before_exit">Discard changes?</string>

    <!--  String displayed when saving and exiting editor [CHAR LIMIT=NONE] -->