doc: shorten overlong title underlines

Title underlines should match the length of the title. Unfortunately
docutils only catches underlines that are too short.

Add some missing empty lines after titles.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/board/ti/k3.rst b/doc/board/ti/k3.rst
index 89d70db..1629d3b 100644
--- a/doc/board/ti/k3.rst
+++ b/doc/board/ti/k3.rst
@@ -238,7 +238,7 @@
 .. k3_rst_include_end_board_env_vars_desc
 
 Building tiboot3.bin
-^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^
 
 1. To generate the U-Boot SPL for the wakeup domain, use the following
    commands, substituting :code:`{SOC}` for the name of your device (eg:
@@ -273,7 +273,7 @@
    UBoot SPL will only look for and load the files with these names.
 
 Building tispl.bin
-^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^
 
 The `tispl.bin` is a standard fitImage combining the firmware need for
 the main domain to function properly as well as Device Management (DM)