fix stale reference to empty priority color config

color.pri.none is now color.uda.priority.none

doc/man/task-color.5.in

@@ -227,8 +227,8 @@ rule 'color.deleted' has the highest precedence, and 'color.uda.' the lowest.
 The keyword rule shown here as 'keyword.' corresponds to a wildcard pattern,
 meaning 'color.keyword.*', or in other words all the keyword rules.
 
-There is also 'color.project.none', 'color.tag.none' and 'color.pri.none' to
-specifically represent missing data.
+There is also 'color.project.none', 'color.tag.none' and
+'color.uda.priority.none' to specifically represent missing data.
 
 .SH THEMES
 Taskwarrior supports themes.  What this really means is that with the ability to