Documentation: PM: Fix runtime_pm.rst markdown syntax

The '7. Generic subsystem callbacks' should be a section title, but
the markdown syntax is not adequate now.

Fix it so the chapter can be linked at docs.kernel.org correctly.

Signed-off-by: Yiwei Lin <s921975628@gmail.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>