Doc: Fix a formatting glitch

While converting the interrupt framework design document from Markdown
to reStructuredText format, see commit 6f625747

 ("Convert
documentation to reStructuredText"), the structure of the document was
changed and one of the titles got lost. Fix that.

Change-Id: Ib3e6c1f9f01f16108fde3b89ceb624bd54049d2f
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

docs/interrupt-framework-design.rst

@@ -954,11 +954,11 @@ The TSP handles interrupts under the asynchronous model as follows.
    control to the TSP in response to an SMC with ``TSP_FID_RESUME`` as the
    function identifier from the non-secure state (see section 2.3.2.4).
 
-#. .. rubric:: Other considerations
-      :name: other-considerations
+Other considerations
+--------------------
 
 Implication of preempted SMC on Non-Secure Software
----------------------------------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 A ``yielding`` SMC call to Secure payload can be preempted by a non-secure
 interrupt and the execution can return to the non-secure world for handling
@@ -1001,7 +1001,7 @@ TSP by returning ``SMC_UNK`` error.
 
 --------------
 
-*Copyright (c) 2014-2018, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2014-2019, Arm Limited and Contributors. All rights reserved.*
 
 .. _Porting Guide: ./porting-guide.rst
 .. _SMC calling convention: http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html