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/j721e_evm.rst b/doc/board/ti/j721e_evm.rst
index cbb7da6..5f14e45 100644
--- a/doc/board/ti/j721e_evm.rst
+++ b/doc/board/ti/j721e_evm.rst
@@ -111,7 +111,8 @@
 .. j721e_evm_rst_include_end_build_steps
 
 Target Images
---------------
+-------------
+
 In order to boot we need tiboot3.bin, sysfw.itb, tispl.bin and u-boot.img.
 Each SoC variant (GP, HS-FS and HS-SE) requires a different source for these
 files.