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>

doc/api/n-api.md

@@ -1590,7 +1590,7 @@ x is passed in it returns `napi_string_expected`.
 
 This API returns the UTF8-encoded string corresponding the value passed in.
 
-#### *napi_get_value_string_utf16_length*
+#### *napi_get_value_string_utf16*
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