pack-heuristics.txt: mark up the file header properly

AsciiDoc wants these header-lines left-aligned.
Signed-off-by: NThomas Ackermann <th.acker@arcor.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/technical/pack-heuristics.txt

-        Concerning Git's Packing Heuristics
-        ===================================
+Concerning Git's Packing Heuristics
+===================================
 
         Oh, here's a really stupid question: