Correct default value for BuildInParallel

[steffenloesch]

According to the MSBuild task documentation (https://docs.microsoft.com/en-us/visualstudio/msbuild/msbuild-task) the default value for BuildInParallel is false.

[msftclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request.
Thanks,
Microsoft Pull Request Bot

[saldana]

✅ Validation status: passed

File	Status	Preview URL	Details
docs/msbuild/building-multiple-projects-in-parallel-with-msbuild.md	✅Succeeded

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[steffenloesch]

Interestingly, the default for the BuildInParallel project property to true (https://docs.microsoft.com/en-us/visualstudio/msbuild/common-msbuild-project-properties), while the default for the BuildInParallel task parameter of the MSBuild task is false. Some confusion might come from that.