Documentation: fix section header mark-up

Asciidoctor is stricter than AsciiDoc when deciding if underlining
is a section title or the start of preformatted text.  Make the
length of the underlining match the text to ensure that it renders
correctly in all implementations.

Signed-off-by: John Keeping <john@keeping.me.uk>
[jc: squashed in git-bisect one noticed by Michael J Gruber]
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-bisect-lk2009.txt b/Documentation/git-bisect-lk2009.txt
index afeb86c..78b1065 100644
--- a/Documentation/git-bisect-lk2009.txt
+++ b/Documentation/git-bisect-lk2009.txt
@@ -1321,7 +1321,7 @@
 _____________
 
 Acknowledgments
-----------------
+---------------
 
 Many thanks to Junio Hamano for his help in reviewing this paper, for
 reviewing the patches I sent to the Git mailing list, for discussing
diff --git a/Documentation/user-manual.txt b/Documentation/user-manual.txt
index 46aa6bc..c964a8b 100644
--- a/Documentation/user-manual.txt
+++ b/Documentation/user-manual.txt
@@ -3424,7 +3424,7 @@
 
 [[the-index]]
 The index
------------
+---------
 
 The index is a binary file (generally kept in `.git/index`) containing a
 sorted list of path names, each with permissions and the SHA-1 of a blob