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/usage/measured_boot.rst b/doc/usage/measured_boot.rst
index 0aad590..9691904 100644
--- a/doc/usage/measured_boot.rst
+++ b/doc/usage/measured_boot.rst
@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0+
 
 Measured Boot
-=====================
+=============
 
 U-Boot can perform a measured boot, the process of hashing various components
 of the boot process, extending the results in the TPM and logging the
@@ -16,7 +16,7 @@
 against the hash results of previous boots.
 
 Requirements
----------------------
+------------
 
 * A hardware TPM 2.0 supported by the U-Boot drivers
 * CONFIG_TPM=y