Typo in RelNotes.

Signed-off-by: NMikael Magnusson <mikachu@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/RelNotes-1.5.6.txt

@@ -70,7 +70,7 @@ Updates since v1.5.5
 
 * "git init --bare" is a synonym for "git --bare init" now.
 
-* "git gc --auto" honors a new pre-aut-gc hook to temporarily disable it.
+* "git gc --auto" honors a new pre-auto-gc hook to temporarily disable it.
 
 * "git log --pretty=tformat:<custom format>" gives a LF after each entry,
   instead of giving a LF between each pair of entries which is how