new-command.txt: correct the command description file

It has always been command-list.txt even at the time this
new-command.txt document is added.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/howto/new-command.txt

@@ -94,7 +94,7 @@ your language, document it in the INSTALL file.
 6. There is a file command-list.txt in the distribution main directory
 that categorizes commands by type, so they can be listed in appropriate
 subsections in the documentation's summary command list.  Add an entry
-for yours.  To understand the categories, look at git-commands.txt
+for yours.  To understand the categories, look at command-list.txt
 in the main directory.  If the new command is part of the typical Git
 workflow and you believe it common enough to be mentioned in 'git help',
 map this command to a common group in the column [common].