Fix typo in documentation page title

Change-Id: I426ffc8717757e35e556f675162a729ba095b7d5
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>

docs/psci-pd-tree.rst

-PSCI Library Integration guide for Armv8-A AArch32 systems
-==========================================================
+PSCI Power Domain Tree design
+=============================
 
 
 .. section-numbering::