Doc: Adjusted tfm_secure_irq_handling document headers

This patch modifies the header levels for the
tfm_secure_irq_handling document, to match the rest of the
documentation.

Change-Id: Id0b6f7156d4001941e1062fdea68e9c7f1bcf88d
Signed-off-by: Galanakis, Minos <minos.galanakis@arm.com>
diff --git a/docs/user_guides/tfm_secure_irq_handling.rst b/docs/user_guides/tfm_secure_irq_handling.rst
index 970d6ba..bde9a25 100644
--- a/docs/user_guides/tfm_secure_irq_handling.rst
+++ b/docs/user_guides/tfm_secure_irq_handling.rst
@@ -86,9 +86,8 @@
 ``void tfm_enable_irq(psa_signal_t irq_signal)``. The function can be called in
 the service init function.
 
-*************
 Library model
-*************
+=============
 
 In Library model a function with the name derived from the value of the
 ``source`` property is generated. This function will be put in the vector table
@@ -112,21 +111,19 @@
 model see
 `Secure Partition Interrupt Handling design document <https://developer.trustedfirmware.org/w/tf_m/design/secure_partition_interrupt_handling/>`_.
 
-*********
 IPC model
-*********
+=========
 
 The detailed description on how secure interrupt handling works in the IPC
 model, see the
 `PSA Firmware Framework and RoT Services specification <https://pages.arm.com/psa-resources-ff.html>`_.
 
-######################
+**********************
 Implementation details
-######################
+**********************
 
-****************************
 Library model implementation
-****************************
+============================
 
 As a result of the function call like behaviour of secure services in library
 model, some information that is critical for the SPM to keep track of partition