Fix documentation of available engines for multinom_reg()

[hfrick]

The documentation said there was a Stan engine but it's Spark:

parsnip::show_engines("multinom_reg")
#> # A tibble: 4 x 2
#>   engine mode          
#>   <chr>  <chr>         
#> 1 glmnet classification
#> 2 spark  classification
#> 3 keras  classification
#> 4 nnet   classification

^{Created on 2021-02-23 by the reprex package (v0.3.0)}

[juliasilge]

Fantastic! 🚀

[github-actions]

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