MyFirstObjectWalk: fix misspelled "builtins/"

pack-objects.c resides in builtin/ (not builtins/).

Fix the misspelled directory name.

Signed-off-by: Dirk Gouders <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dgouders

Documentation/MyFirstObjectWalk.txt

@@ -525,7 +525,7 @@ about each one.
 
 We can base our work on an example. `git pack-objects` prepares all kinds of
 objects for packing into a bitmap or packfile. The work we are interested in
-resides in `builtins/pack-objects.c:get_object_list()`; examination of that
+resides in `builtin/pack-objects.c:get_object_list()`; examination of that
 function shows that the all-object walk is being performed by
 `traverse_commit_list()` or `traverse_commit_list_filtered()`. Those two
 functions reside in `list-objects.c`; examining the source shows that, despite