Tweak example in configuration combinations

Swaps the platform for one project configuration in the  Configuration X Platform matrix example, so it's clearer why the first set requires the existence of the second set. Otherwise, it's not obvious why three Win32 configurations require three x64 configurations (but not three ARM configurations).

Author: dmringo

docs/build/reference/vcxproj-file-structure.md

@@ -126,15 +126,15 @@ The IDE expects to find a project configuration for any combination of `Configur
 
 - Debug|Win32
 
-- Retail|Win32
+- Retail|x64
 
 - Special 32-bit Optimization|Win32
 
 then it must also have these configurations, even though "Special 32-bit Optimization" is meaningless for x64:
 
 - Debug|x64
 
-- Retail|x64
+- Retail|Win32
 
 - Special 32-bit Optimization|x64