git-bisect.txt: BISECT_HEAD is not that special

The description of "git bisect --no-checkout" called BISECT_HEAD a
"special ref", but there is nothing special about it.  It merely is
yet another pseudoref.

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

Documentation/git-bisect.txt

@@ -362,7 +362,7 @@ OPTIONS
 --no-checkout::
 +
 Do not checkout the new working tree at each iteration of the bisection
-process. Instead just update a special reference named `BISECT_HEAD` to make
+process. Instead just update the reference named `BISECT_HEAD` to make
 it point to the commit that should be tested.
 +
 This option may be useful when the test you would perform in each step