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/tools/binman/entries.rst b/tools/binman/entries.rst
index 2402adb..61de7f1 100644
--- a/tools/binman/entries.rst
+++ b/tools/binman/entries.rst
@@ -1,5 +1,5 @@
 Binman Entry Documentation
-===========================
+==========================
 
 This file describes the entry types supported by binman. These entry types can
 be placed in an image one by one to build up a final firmware image. It is