Fix Documentation for git-describe

The documentation for git-describe says the default abbreviation is 8
hexadecimal digits while cache.c clearly shows DEFAULT_ABBREV set to 7.
This patch corrects the documentation.

Signed-off-by: Boyd Stephen Smith Jr <bss@iguanasuicide.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -38,7 +38,7 @@ OPTIONS
 	Automatically implies --tags.
 
 --abbrev=<n>::
-	Instead of using the default 8 hexadecimal digits as the
+	Instead of using the default 7 hexadecimal digits as the
 	abbreviated object name, use <n> digits.
 
 --candidates=<n>::