sstate-cache-management.sh:fix the incorrect usage of option `-d'

sstate-cache-management.sh:fix the incorrect usage of option `-d'

The description of option `-d' is not correct in useage. It is used to
remove duplicate and debug at the same time. Use option `-D' to control
debug info output and the option `-d' to flag remove duplicate.

[YOCTO #3635]

(From OE-Core rev: fa0b40c233e757fe986aa45798b35b60b89c879f)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>