clean up the postseries texts for the series pages and widget
    
    WE use the description text on a post series  for contructing
    two strings, the text at the top of the post series widget[1]:
    
    This is part x of a x-part series here on Fedora Magazine covering
    
    and the top of the series page[2]:
    
    This is series here on Fedora Magazine
    
    This commit removes the word "covering" from the widget text in the
    template, so its easier to craft a description that will fit in both
    locations. For example, the new text for the systemd series set in
    the postseries settings would be something like:
    
    detailing information about the use of SystemD in Fedora
    
    [1] - https://fedoramagazine.org/systemd-template-unit-files/
    [2] - https://fedoramagazine.org/series/systemd-series/