[docs] Fix formula in path smoothing docs (fixes #5139)

[samFarrellDay]

fixes #5139

Very simple change, just 1 line to docs.

[ghost]

All CLA requirements met.

[jameslamb]

Thanks very much for taking the time to improve LightGBM! This change looks like it matches the corresponding code that was linked from #5139.

[jameslamb]

Don't worry about the failing CI jobs. They're unrelated to this PR, and I have a fix up in #5152 to fix them. I'll update this PR after that's merged.

By the way...in the future when you make contributions here or in other open source projects, I strongly recommend using a dedicated branch on your fork, instead of opening a pull request directly from master / main. Once this PR is merged, since we use squash merging here master of this repo will have a conflicting commit history with your fork.

[guolinke]

close and reopen to rerun CI

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.