doc/rebase -i: fix typo in the documentation of 'fixup' command

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -894,7 +894,7 @@ is used.  In that case the suggested commit message is only the message
 of the "fixup -c" commit, and an editor is opened allowing you to edit
 the message.  The contents (patch) of the "fixup -c" commit are still
 incorporated into the folded commit. If there is more than one "fixup -c"
-commit, the message from the last last one is used.  You can also use
+commit, the message from the final one is used.  You can also use
 "fixup -C" to get the same behavior as "fixup -c" except without opening
 an editor.