Fixed a typo in system barycenter description

[Ranganrs1]

"exact of mass" -> "exact center of mass" Fixes #13

[DasSkelett]

You can put "Fixes #13" or "Closes #13" into the pull request description, and the issue will automatically be closed once this PR gets merged.

https://docs.github.com/en/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword

[Ranganrs1]

You can put "Fixes #13" or "Closes #13" into the pull request description, and the issue will automatically be closed once this PR gets merged.

https://docs.github.com/en/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword

Ah, thank you very much! Wasn't aware of that.