Documentation: match undefline with the text in old release notes

These are not processed with AsciiDoc, but it is better to be
consistent.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/RelNotes/1.8.3.1.txt

 Git v1.8.3.1 Release Notes
-========================
+==========================
 
 Fixes since v1.8.3
 ------------------

Documentation/RelNotes/1.8.4.1.txt

 Git v1.8.4.1 Release Notes
-========================
+==========================
 
 Fixes since v1.8.4
 ------------------

Documentation/RelNotes/1.8.4.2.txt

 Git v1.8.4.2 Release Notes
-========================
+==========================
 
 Fixes since v1.8.4.1
 --------------------

Documentation/RelNotes/1.8.4.3.txt

 Git v1.8.4.3 Release Notes
-========================
+==========================
 
 Fixes since v1.8.4.2
 --------------------

Documentation/RelNotes/1.8.4.4.txt

 Git v1.8.4.4 Release Notes
-========================
+==========================
 
 Fixes since v1.8.4.3
 --------------------