navigate march: Say "to sweep mines" instead of "to minesweep"

navigate march: Say "to sweep mines" instead of "to minesweep"

The choice of "to minesweep" in "`d' to drop mines, and `m' to
minesweep" is obviously intentional.  But saying it in standard
English instead is at least as clear, so do that.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>