syntax: avoid using black colors

We should avoid defining color schemes that are unreadable on common
white-on-black or black-on-white terminals.  Change the black in the
autoconf error strings to brightred for that reason.

doc/syntax/autoconf.nanorc

@@ -20,7 +20,7 @@ color red "\"[^"]*\""  "\'[^']*\'"
 color green "`[^`]*`"
 
 # Error lines:
-color black "^\*\*\*.*"
+color brightred "^\*\*\*.*"
 
 # Brackets:
 color magenta "\[|\]|\(|\)"