doc: fix title/function name mismatch

Fix mismatch in title for napi_get_value_string_utf16

Fixes: https://github.com/nodejs/abi-stable-node/issues/243
PR-URL: https://github.com/nodejs/node/pull/13123


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>