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/cmd/bootdev.rst b/doc/usage/cmd/bootdev.rst
index 6c68d0b..fb638b5 100644
--- a/doc/usage/cmd/bootdev.rst
+++ b/doc/usage/cmd/bootdev.rst
@@ -76,7 +76,7 @@
 
 
 bootdev select
-~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~
 
 Use this to select a particular bootdev. You can select it by the sequence
 number or name, as shown in `bootdev list`.
@@ -89,7 +89,7 @@
 
 
 bootdev info
-~~~~~~~~~~~~~~~
+~~~~~~~~~~~~
 
 This shows information on the current bootdev, with the format looking like
 this: