Update rootfs.md: Fix formatting and wording of idmap option

The newly introduced `idmap` section of rootfs lacked a header
(comparable to Overlay Rootfs Mounts), had odd formatting, and
wording that differed from other instances of idmap, e.g., the
one in the --volume section. This commits addresses those issues.

Signed-off-by: Peter Whittaker <PeterWhittaker@SphyrnaSecurity.com>

docs/source/markdown/options/rootfs.md

@@ -22,7 +22,9 @@ finishes executing, similar to a tmpfs mount point being unmounted.
 Note: On **SELinux** systems, the rootfs needs the correct label, which is by default
 **unconfined_u:object_r:container_file_t:s0**.
 
-    The `idmap` option if specified, creates an idmapped mount to the target user
+  `idmap`
+
+If `idmap` is specified, create an idmapped mount to the target user
 namespace in the container.
 The idmap option supports a custom mapping that can be different than the user
 namespace used by the container.  The mapping can be specified after the idmap