doc: fix n-api example string

- using a length of 6 for `"hello"` includes a terminating null yielding
  `"hello\u0000"`
- length of 5 would work,
but comparing to the N-API docs gives `NAPI_AUTO_LENGTH` instead

PR-URL: https://github.com/nodejs/node/pull/19205
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/api/addons.md

@@ -246,7 +246,7 @@ napi_value Method(napi_env env, napi_callback_info args) {
   napi_value greeting;
   napi_status status;
 
-  status = napi_create_string_utf8(env, "hello", 6, &greeting);
+  status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &greeting);
   if (status != napi_ok) return nullptr;
   return greeting;
 }