Documentation: PM: Fix a reStructuredText syntax error

Fix a reStructuredText syntax error in the cpuidle PM admin-guide
documentation: the ``...'' quotation marks are parsed as partial ''...''
reStructuredText markup and break the output formatting.

This change them to "...".

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>