binman: Update the entrydocs header

Reduce the length of the underline for this header, to match the
heading itself.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index 42e0b7b..2ed6580 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -812,7 +812,7 @@
                 as missing
         """
         print('''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