doc: correct a mistake in an illustration

doc: correct a mistake in an illustration

The first illustration of the "RECOVERING FROM UPSTREAM REBASE"
section in the 'git-rebase' documentation meant to depict that
there are number of commits on the 'master' branch, but it is
longer than the 'master' branch in the following illustrations
by one commit, even though there is no resetting of 'master' to
lose that commit.

Correct it.

Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>