bpo-31569: correct PCBuild/ case to PCbuild/ in build scripts and docs (GH-3711)

diff --git a/Tools/nuget/build.bat b/Tools/nuget/build.bat
index 0b4f622..f75cb3f 100644
--- a/Tools/nuget/build.bat
+++ b/Tools/nuget/build.bat
@@ -1,7 +1,7 @@
 @echo off
 setlocal
 set D=%~dp0
-set PCBUILD=%D%..\..\PCBuild\
+set PCBUILD=%D%..\..\PCbuild\
 if "%Py_OutDir%"=="" set Py_OutDir=%PCBUILD%
 
 set BUILDX86=