Documentation: fix header markup

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: Xue Fuqiao <xfq.free@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -1491,7 +1491,7 @@ resolving a merge>>.
 
 [[fixing-a-mistake-by-rewriting-history]]
 Fixing a mistake by rewriting history
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 If the problematic commit is the most recent commit, and you have not
 yet made that commit public, then you may just