Documentation: PM: amd-pstate: Fix section title underline

Title under line too short

Signed-off-by: Meng Li <li.meng@amd.com>
[ rjw: Subject edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: Meng Li

Documentation/admin-guide/pm/amd-pstate.rst

@@ -381,7 +381,7 @@ driver receives a message with the highest performance change, it will
 update the core ranking and set the cpu's priority.
 
 ``amd-pstate`` Preferred Core Switch
-=================================
+=====================================
 Kernel Parameters
 -----------------