docs: RCU: Requirements.rst: Fix a list block

As warned by Sphinx:
	.../Documentation/RCU/Design/Requirements/Requirements.rst:1959: WARNING: Unexpected indentation.

The list block is missing a space before it, making Sphinx to get
it wrong.  This commit therefore adds the missing space characters.

Fixes: 2a721e5f0b2c ("docs: Update RCU's hotplug requirements with a bit about design")
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>

Documentation/RCU/Design/Requirements/Requirements.rst

@@ -1954,6 +1954,7 @@ offline CPUs.  However, as a debugging measure, the FQS loop does splat
 if offline CPUs block an RCU grace period for too long.
 
 An offline CPU's quiescent state will be reported either:
+
 1.  As the CPU goes offline using RCU's hotplug notifier (``rcu_report_dead()``).
 2.  When grace period initialization (``rcu_gp_init()``) detects a
     race either with CPU offlining or with a task unblocking on a leaf