docs/system/pr-manager.rst: Fix minor docs nits

Fix a couple of nits in pr-manager.rst:
 * the title marker for the top level heading is overlength
 * stray capital 'R' in the middle of a sentence

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

docs/system/pr-manager.rst

@@ -1,8 +1,8 @@
-======================================
+===============================
 Persistent reservation managers
-======================================
+===============================
 
-SCSI persistent Reservations allow restricting access to block devices
+SCSI persistent reservations allow restricting access to block devices
 to specific initiators in a shared storage setup.  When implementing
 clustering of virtual machines, it is a common requirement for virtual
 machines to send persistent reservation SCSI commands.  However,