f2fs: fix spelling in ABI documentation

Correct spelling problems as identified by codespell.

Fixes: 9e615dbb ("f2fs: add missing description for ipu_policy node")
Fixes: b2e4a2b3 ("f2fs: expose discard related parameters in sysfs")
Fixes: 846ae671 ("f2fs: expose extension_list sysfs entry")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Chao Yu <chao@kernel.org>
Cc: linux-f2fs-devel@lists.sourceforge.net
Cc: Yangtao Li <frank.li@vivo.com>
Cc: Konstantin Vyshetsky <vkon@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>