doc: remove undef NDEBUG from addons.md

When working on commit 0d95a0b60a
("test: remove undef NDEBUG from at-exit addons test) I searched for
usages of undef NDEBUG but did not include the doc directory (but I did
include the test directory) and missed this one.

Commit 1f02569f8db9cb0101807df4982534738f0161b2 ("tools: fix
tools/addon-verify.js") enables the code in the "AtExit" section to be
included in the test/addons diretory and this code will again be
tested.

PR-URL: https://github.com/nodejs/node/pull/14048
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api/addons.md

@@ -1053,7 +1053,6 @@ The following `addon.cc` implements AtExit:
 
 ```cpp
 // addon.cc
-#undef NDEBUG
 #include <assert.h>
 #include <stdlib.h>
 #include <node.h>