Documentation/git-rebase.txt: <upstream> must be given to specify <branch>

Current syntax description makes one wonder if there is any
syntactic way to distinguish between <branch> and <upstream> so that
one can specify <branch> but not <upstream>, but that is not the
case.

Make it explicit that these arguments are positional, i.e. the
earlier ones cannot be omitted if you want to give later ones.
Signed-off-by: NSergey Organov <sorganov@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git rebase' [-i | --interactive] [options] [--exec <cmd>] [--onto <newbase>]
-	[<upstream>] [<branch>]
+	[<upstream> [<branch>]]
 'git rebase' [-i | --interactive] [options] [--exec <cmd>] [--onto <newbase>]
 	--root [<branch>]
 'git rebase' --continue | --skip | --abort | --edit-todo