t3404: fix typo

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

t/t3404-rebase-interactive.sh

@@ -62,7 +62,7 @@ test_expect_success 'setup' '
 
 # "exec" commands are ran with the user shell by default, but this may
 # be non-POSIX. For example, if SHELL=zsh then ">file" doesn't work
-# to create a file. Unseting SHELL avoids such non-portable behavior
+# to create a file. Unsetting SHELL avoids such non-portable behavior
 # in tests. It must be exported for it to take effect where needed.
 SHELL=
 export SHELL