Fix minor typo in docstring

[codecov[bot]]

Codecov Report

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

Comparison is base (6ad333b) 85.10% compared to head (34a19cc) 85.10%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #325 +/- ## ======================================= Coverage 85.10% 85.10% ======================================= Files 41 41 Lines 2222 2222 ======================================= Hits 1891 1891 Misses 331 331 ``` | [Files](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/325?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev) | Coverage Δ | | |---|---|---| | [src/Bridges/Constraint/sos\_polynomial.jl](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/325?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev#diff-c3JjL0JyaWRnZXMvQ29uc3RyYWludC9zb3NfcG9seW5vbWlhbC5qbA==) | `100.00% <ø> (ø)` | | | [.../Constraint/sos\_polynomial\_in\_semialgebraic\_set.jl](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/325?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev#diff-c3JjL0JyaWRnZXMvQ29uc3RyYWludC9zb3NfcG9seW5vbWlhbF9pbl9zZW1pYWxnZWJyYWljX3NldC5qbA==) | `80.55% <ø> (ø)` | | | [src/attributes.jl](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/325?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev#diff-c3JjL2F0dHJpYnV0ZXMuamw=) | `73.68% <ø> (ø)` | |

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