VS: Fix .vcxproj ProjectGuid element case

Brad Kingrequested to merge
brad.king/cmake:vs-project-guid into master

The .vcxproj file format expects ProjectGuid, not ProjectGUID. The latter is expected by .vcproj files from VS 2008, so this was likely a typo when the VS 2010 generator was first introduced.

Fixes: #11968 (closed)

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