Fix typo in constrained_outer_boundary.jl docs

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.06%. Comparing base (8b8aef2) to head (cb6c630).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #114 +/- ## ======================================= Coverage 95.06% 95.06% ======================================= Files 92 92 Lines 9095 9095 ======================================= Hits 8646 8646 Misses 449 449 ```

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