src/help.c: Update messages referring to durations
src/help.c: Update messages referring to durations

Fix help texts to vaguely match the new duration string formats.

Signed-off-by: Lukas Fleischer <af90e31281d8305af43dc616c5aaca3c7165f8a7@cryptocrack.de>

lfos/calcurse,lfos/calcurse,lfos/calcurse