Fix --disable-cache help message.
Fix --disable-cache help message.

In its first iteration, a command-line switch was used to turn
caching on. Commit 93dc6d1 modified the command-line switch to
turn caching off (i.e., caching became on by default). The help
message was accidentally not updated in that commit, which this
commit fixes.

davidalber/Geneagrapher,davidalber/Geneagrapher