man: fix missing description for overprovision ratio
man: fix missing description for overprovision ratio

This patch updates mkfs.f2fs man page to indicate the automatic ovp
calculation.

Change-Id: I8558b689720962f1ccc77edffbe602fe5a1da148
Signed-off-by: Jaegeuk Kim <71255387a32c360dfbe6dfb7aaeb123bf1dc8cb6@kernel.org>

blastagator/cm_external_f2fs-tools,blastagator/cm_external_f2fs-tools,PAC-ROM/android_external_f2fs-tools,NamelessRom/android_external_f2fs-tools,TeamExodus/external_f2fs-tools,TeamExodus/external_f2fs-tools,NamelessRom/android_external_f2fs-tools,PAC-ROM/android_external_f2fs-tools