git-rebase--interactive: fix copy-paste mistake

exec argument is a command, not a commit.

Signed-off-by: Orgad Shaneh <orgads@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -160,7 +160,7 @@ r, reword <commit> = use commit, but edit the commit message
 e, edit <commit> = use commit, but stop for amending
 s, squash <commit> = use commit, but meld into previous commit
 f, fixup <commit> = like \"squash\", but discard this commit's log message
-x, exec <commit> = run command (the rest of the line) using shell
+x, exec <command> = run command (the rest of the line) using shell
 d, drop <commit> = remove commit
 l, label <label> = label current HEAD with a name
 t, reset <label> = reset HEAD to a label