Give a meaning to DATE_FORMATS

It's a bit weird to consider DATE_FORMATS as part of the date input
format enum, all the more so as it does not even represent the number
of available formats (since those are numbered from 1).

Turn DATE_FORMATS into the number of available date input formats.

Signed-off-by: Baptiste Jonglez <baptiste--git@jonglez.org>
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/custom.c

@@ -792,7 +792,7 @@ void custom_general_config(void)
     case '0':
       val = status_ask_simplechoice(input_datefmt_prefix,
                                     input_datefmt_choices,
-                                    DATE_FORMATS - 1);
+                                    DATE_FORMATS);
       if (val != -1)
         conf.input_datefmt = val;
       break;