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>