docs(porting-guide): update a reference

The BL31 part has been there forever and the PSCI reference is neither
at section 3.3 or directly below. Update this to locate the section more
easily.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I9a86e4ef13d1ac5da743917493f63ddd7690e087

docs/getting_started/porting-guide.rst

@@ -2135,7 +2135,7 @@ CPUs. BL31 executes at EL3 and is responsible for:
 
 #. Providing runtime firmware services. Currently, BL31 only implements a
    subset of the Power State Coordination Interface (PSCI) API as a runtime
-   service. See Section 3.3 below for details of porting the PSCI
+   service. See :ref:`psci_in_bl31` below for details of porting the PSCI
    implementation.
 
 #. Optionally passing control to the BL32 image, pre-loaded at a platform-
@@ -2544,6 +2544,8 @@ Function: bool plat_get_entropy(uint64_t \*out) [mandatory]
 This function writes entropy into storage provided by the caller. If no entropy
 is available, it must return false and the storage must not be written.
 
+.. _psci_in_bl31:
+
 Power State Coordination Interface (in BL31)
 --------------------------------------------