man: fix description for rootfs

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>

Author: Ma Shimiao

man/ocitools-generate.1.md

@@ -168,7 +168,9 @@ read the configuration from `config.json`.
   Values are "shared, rshared, private, rprivate, slave, rslave"
 
 **--rootfs**=ROOTFSPATH
-  Path to the rootfs
+  Path to the rootfs, which is relative to bundle path.
+  e.g the absolute path of rootfs is /to/bundle/rootfs, bundle path is /to/bundle,
+  then the value set as ROOTFSPATH should be rootfs.
 
 **--seccomp-arch**=ARCH
   Specifies Additional architectures permitted to be used for system calls.