ug: custom-ci: Fix the method to grab ostree hash

The command cat ./deploy/images/intel-corei7-64/ostree_repo/refs/heads/<refs>
Returns the 64-characters SHA256 checksum required to use with fioctl
targets add command.

The command find ./deploy/images/intel-corei7-64/ostree_repo -name
*.commit provides the entire metadata + 64 characters
SHA256.

When this value is used in the fioctl targets add command ,
it causes ERROR: Illegal base64 data at input byte 60.

Signed-off-by: Caio Pereira <[email protected]>

Author: caiotpereira

source/user-guide/custom-ci/custom-ci-for-rootfs.rst

@@ -66,7 +66,7 @@ Add OSTree Target
 Once the ostree repo carrying rootfs is pushed to the cloud, you can add a new Target referencing it.
 
 The rootfs committed to the ostree repo is referenced by the commit hash.
-To obtain, run ``find ./deploy/images/intel-corei7-64/ostree_repo -name *.commit``
+To obtain, run ``cat ./deploy/images/intel-corei7-64/ostree_repo/refs/heads/<refs>``
 or ``ostree --repo ./deploy/images/intel-corei7-64/ostree_repo rev-parse <machine|hardware ID>``.
 
 Run ``fioctl targets add`` ato add the new Target referencing the given ostree-based rootfs, e.g.,