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

AsciiDoc wants these header-lines left-aligned.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/pack-heuristics.txt

@@ -1,5 +1,5 @@
-        Concerning Git's Packing Heuristics
-        ===================================
+Concerning Git's Packing Heuristics
+===================================
 
         Oh, here's a really stupid question: