mkfs.f2fs: fix wrong documentation
mkfs.f2fs: fix wrong documentation

The -s should be the number of segments per a section.

Reported-by: Marc Lehmann <16b244ffbfb15aaa5cdf4c0b3b307cc56ad2be79@schmorp.de>
Signed-off-by: Jaegeuk Kim <71255387a32c360dfbe6dfb7aaeb123bf1dc8cb6@kernel.org>

temasek/android_external_f2fs-tools,Oi-Android/android_external_f2fs-tools,Oi-Android/android_external_f2fs-tools,Khaon/android_external_f2fs-tools,Khaon/android_external_f2fs-tools,temasek/android_external_f2fs-tools