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.

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

temasek/android_external_f2fs-tools,temasek/android_external_f2fs-tools