t/: correct obvious typo "detahced"

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7409-submodule-detached-work-tree.sh

@@ -6,7 +6,7 @@
 test_description='Test submodules on detached working tree
 
 This test verifies that "git submodule" initialization, update and addition works
-on detahced working trees
+on detached working trees
 '
 
 TEST_NO_CREATE_REPO=1