songs: rename keyword cov -> cover for compat with latest patacrep version

tex/crepbook.cls

@@ -203,7 +203,7 @@
 \setlength{\coverspace}{0.1cm}
 \newcommand{\songcover}{}
 \newcommand{\songalbum}{}
-\newsongkey{cov}{\let\songcover\@empty}{\def\songcover{\currfiledir#1}}
+\newsongkey{cover}{\let\songcover\@empty}{\def\songcover{\currfiledir#1}}
 \newsongkey{album}{\let\songalbum\@empty}{\def\songalbum{#1}}
 \newsongkey{url}{\let\songurl\@empty}{\def\songurl{#1}}
 \newsongkey{original}{\let\songoriginal\@empty}{\def\songoriginal{#1}}