describe doc: remove '7-char' abbreviation reference

While the minimum is 7-char, the unambiguous length can be longer.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -139,7 +139,7 @@ at the end.
 
 The number of additional commits is the number
 of commits which would be displayed by "git log v1.0.4..parent".
-The hash suffix is "-g" + 7-char abbreviation for the tip commit
+The hash suffix is "-g" + unambiguous abbreviation for the tip commit
 of parent (which was `2414721b194453f058079d897d13c4e377f92dc6`).
 The "g" prefix stands for "git" and is used to allow describing the version of
 a software depending on the SCM the software is managed with. This is useful