Fix syntax in C++ Core Guidelines MSBuild example

[chwarr]

There was an unclosed XML element in the BuildMacro example.

[msftclas]

@chwarr,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[mikeblome]

thanks!

[saldana]

✅ Validation status: passed

File	Status	Preview URL	Details
docs/code-quality/using-the-cpp-core-guidelines-checkers.md	✅Succeeded

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.

[saldana]

✅ Validation status: passed

File	Status	Preview URL	Details
docs/code-quality/using-the-cpp-core-guidelines-checkers.md	✅Succeeded

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.