pure-ftpd: Config.in: fix help text wrapping

This commit fixes the warnings reported by check-package on the help
text of package Config.in file, related to the formatting of the help
text: should start with a tab, then 2 spaces, then at most 62
characters.

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Acked-by: Sam Voss <sam.voss@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>