HPCC-11858 DOCS: DFUPlus copy files

FIX HPCC-11858 DOCS: DFUPlus copy files
Addresses the Client tools manual for dfuplus
should mention dropping files on the drop zone
prior to trying to spray the files. Clarify that
files need to be copied first.

Signed-off-by: g-pan <greg.panagiotatos@lexisnexis.com>

@JamesDeFabia please review