VS: Fix .vcxproj ProjectGuid element case

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

Source/cmGlobalVisualStudio10Generator.cxx

@@ -647,9 +647,9 @@ bool cmGlobalVisualStudio10Generator::FindVCTargetsPath(cmMakefile* mf)
      xw.EndElement(); // ItemGroup
     xw.StartElement("PropertyGroup");
      xw.Attribute("Label", "Globals");
-     xw.StartElement("ProjectGUID");
+     xw.StartElement("ProjectGuid");
       xw.Content("{F3FC6D86-508D-3FB1-96D2-995F08B142EC}");
-     xw.EndElement(); // ProjectGUID
+     xw.EndElement(); // ProjectGuid
      xw.StartElement("Keyword");
       xw.Content("Win32Proj");
      xw.EndElement(); // Keyword

Source/cmVisualStudio10TargetGenerator.cxx

@@ -297,8 +297,8 @@ void cmVisualStudio10TargetGenerator::Generate()
     this->WriteProjectConfigurations();
   }
   this->WriteString("<PropertyGroup Label=\"Globals\">\n", 1);
-  this->WriteString("<ProjectGUID>", 2);
-  (*this->BuildFileStream) << "{" << this->GUID << "}</ProjectGUID>\n";
+  this->WriteString("<ProjectGuid>", 2);
+  (*this->BuildFileStream) << "{" << this->GUID << "}</ProjectGuid>\n";
 
   if (this->MSTools &&
       this->GeneratorTarget->GetType() <= cmStateEnums::GLOBAL_TARGET) {