build: make doc generation work on Windows

PR-URL: #22749
Refs: #21697
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

vcbuild.bat

@@ -459,7 +459,7 @@ robocopy /e doc\api %config%\doc\api
 robocopy /e doc\api_assets %config%\doc\api\assets
 
 for %%F in (%config%\doc\api\*.md) do (
-  %node_exe% tools\doc\generate.js --node-version=v%FULLVERSION% --analytics=%DOCS_ANALYTICS% %%F --output-dir=%%~dF%%~pF
+  %node_exe% tools\doc\generate.js --node-version=v%FULLVERSION% --analytics=%DOCS_ANALYTICS% %%F --output-directory=%%~dF%%~pF
 )
 
 :run