doc: fix typo
doc: fix typo

Name of last argument to ffi_get_struct_offsets is `offsets`, not `sizes`

Signed-off-by: Yousong Zhou <1921aaedc62853b63cd90883c032392ad74750cc@gmail.com>

plicease/libffi,plicease/libffi,plicease/libffi,plicease/libffi