docs: translate Spanish comment to English in SimpleExponentialSmoothing notebook

[kalichiruvelli-art]

Fixes #1118

Summary

Translates a Spanish comment in the SimpleExponentialSmoothing notebook to English for consistency with the rest of the codebase.

Change:

• #plt.savefig("Gráfico de Densidad y qq") → #plt.savefig("Density and QQ Plot")
  This improves accessibility for non-Spanish-speaking contributors and makes the notebook consistent with the project's English-language documentation.

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[codspeed]

Merging this PR will not alter performance

✅ 38 untouched benchmarks

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_{Comparing kalichiruvelli-art:fix/remove-spanish-comments (39c8ccf) with main (9d3b2a7)}

[Saul (nasaul)]

Thanks and congratulations on your first contribution :)