Fix a docstring

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (43d9d6d) 88.04% compared to head (b8589be) 88.29%. Report is 10 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #26 +/- ## ========================================== + Coverage 88.04% 88.29% +0.25% ========================================== Files 7 8 +1 Lines 184 188 +4 ========================================== + Hits 162 166 +4 Misses 22 22 ``` | [Files](https://app.codecov.io/gh/JuliaAI/MLJText.jl/pull/26?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaAI) | Coverage Δ | | |---|---|---| | [src/MLJText.jl](https://app.codecov.io/gh/JuliaAI/MLJText.jl/pull/26?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaAI#diff-c3JjL01MSlRleHQuamw=) | `100.00% <ø> (ø)` | | | [src/bm25\_transformer.jl](https://app.codecov.io/gh/JuliaAI/MLJText.jl/pull/26?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaAI#diff-c3JjL2JtMjVfdHJhbnNmb3JtZXIuamw=) | `87.80% <100.00%> (ø)` | | | [src/count\_transformer.jl](https://app.codecov.io/gh/JuliaAI/MLJText.jl/pull/26?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaAI#diff-c3JjL2NvdW50X3RyYW5zZm9ybWVyLmps) | `88.00% <100.00%> (ø)` | | | [src/docstring\_helpers.jl](https://app.codecov.io/gh/JuliaAI/MLJText.jl/pull/26?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaAI#diff-c3JjL2RvY3N0cmluZ19oZWxwZXJzLmps) | `100.00% <100.00%> (ø)` | | | [src/tfidf\_transformer.jl](https://app.codecov.io/gh/JuliaAI/MLJText.jl/pull/26?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaAI#diff-c3JjL3RmaWRmX3RyYW5zZm9ybWVyLmps) | `87.50% <100.00%> (ø)` | |

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