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/mmc.rst b/doc/usage/cmd/mmc.rst
index c0924ba..8394f64 100644
--- a/doc/usage/cmd/mmc.rst
+++ b/doc/usage/cmd/mmc.rst
@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0+:
 
 mmc command
-============
+===========
 
 Synopsis
 --------