doc: fix formatting error in l1tf doc

PR #1714 had one error that this PR is fixing.

Tracked-on: #1672

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

Author: dbkinder

doc/developer-guides/l1tf.rst

@@ -261,6 +261,7 @@ requirements:
 
 5) Put hypervisor security data in UC memory if possible
 6) Enable L1D_FLUSH_VMENTRY option, if
+
    - Doing 5) is not feasible, or
    - CPU sharing is enabled (in the future)