typofix for index-format.txt

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: tacker66

Documentation/technical/index-format.txt

@@ -271,7 +271,7 @@ Git index format
 
     - The directory name terminated by NUL.
 
-    - A number of untrached file/dir names terminated by NUL.
+    - A number of untracked file/dir names terminated by NUL.
 
 The remaining data of each directory block is grouped by type: