Fix libdispatch path in Windows build guide

Author: egorzhdan

docs/WindowsBuild.md

@@ -142,7 +142,7 @@ ninja -C S:\b\toolchain
 ## Running Swift tests on Windows
 
 ```cmd
-path S:\Library\icu-64\usr\bin;S:\b\toolchain\bin;S:\b\toolchain\libdispatch-prefix\bin;%PATH%;%ProgramFiles%\Git\usr\bin
+path S:\Library\icu-64\usr\bin;S:\b\toolchain\bin;S:\b\toolchain\tools\swift\libdispatch-prefix\bin;%PATH%;%ProgramFiles%\Git\usr\bin
 ninja -C S:\b\toolchain check-swift
 ```