doc: fix title/function name mismatch

Fix mismatch in title for napi_get_value_string_utf16

Fixes: nodejs/abi-stable-node#243
PR-URL: #13123
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>

Author: mhdawson

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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