Fix typo in 1.6.6.2 release notes

Of course, these are changes since 1.6.6.1; changes since 1.6.6.2
would have been nil.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes-1.6.6.2.txt

@@ -1,7 +1,7 @@
 Git v1.6.6.2 Release Notes
 ==========================
 
-Fixes since v1.6.6.2
+Fixes since v1.6.6.1
 --------------------
 
  * recursive merge didn't correctly diagnose its own programming errors,