howto/new-command: update reference to builtin docs

Commit ec14d4e (builtin.h: take over documentation from
api-builtin.txt, 2017-08-02) deleted api-builtin.txt and moved the
contents into builtin.h.  Most of the references were fixed in
d85e944 (new-command.txt: update reference to builtin docs,
2023-02-04), but one remained.  Fix it.

Signed-off-by: Todd Zullinger <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: tmzullinger

Documentation/howto/new-command.adoc

@@ -48,7 +48,7 @@ binary); this organization makes it easy for people reading the code
 to find things.
 
 See the CodingGuidelines document for other guidance on what we consider
-good practice in C and shell, and api-builtin.txt for the support
+good practice in C and shell, and builtin.h for the support
 functions available to built-in commands written in C.
 
 What every extension command needs