Update WindowsBuild.md

Deal with different python versions being installed.  GYB is python2 only and breaks with python 3.x.  Ensure that we pick up python 2.7 when building (this is the version vended by Visual Studio and is the stable 2.x series release).

Author: compnerd

docs/WindowsBuild.md

@@ -174,6 +174,7 @@ cmake -G Ninja^
  -DSWIFT_WINDOWS_x86_64_ICU_I18N_INCLUDE="S:/thirdparty/icu4c-63_1-Win64-MSVC2017/include"^
  -DSWIFT_WINDOWS_x86_64_ICU_I18N="S:/thirdparty/icu4c-63_1-Win64-MSVC2017/lib64/icuin.lib"^
  -DCMAKE_INSTALL_PREFIX="C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr"^
+ -DPYTHON_EXECUTABLE=C:\Python27\python.exe^
  S:\swift
 popd
 cmake --build "S:\b\swift"