manyclass_classifier renaming is needed

[merveguel]

Hi all,

Many class extension documentation seems to require a small update on the below line. "manyclass_classifier" module appears to be shortened as "manyclass".

from tabpfn_extensions.manyclass_classifier import ManyClassClassifier

[merveguel]

would also be cool if alphabet_size and n_estimator could be mentioned in the sample, as otherwise the extension isnt working.