fix #518: document fmt param as format string, not function

Agent-Logs-Url: https://github.com/lunarmodules/Penlight/sessions/c46dde20-168b-4b16-9495-0e55bee00fb9

Co-authored-by: Tieske <1077043+Tieske@users.noreply.github.com>

Author: Copilot

lua/pl/seq.lua

@@ -281,15 +281,15 @@ end
 -- @param iter a sequence
 -- @param sep the separator (default space)
 -- @param nfields maximum number of values per line (default 7)
--- @param fmt optional format string or function for each value
+-- @param fmt optional format string for each value
 function seq.printall(iter,sep,nfields,fmt)
   local write = io.write
   if not sep then sep = ' ' end
   if not nfields then
       if sep == '\n' then nfields = 1e30
       else nfields = 7 end
   end
-  if type(fmt) == 'string' then
+  if fmt then
     local fstr = fmt
     fmt = function(v) return format(fstr,v) end
   end