Fixed datasets module references in prepare-wiki.sh

[cosimo]

Had the following errors otherwise:

Saving data in data
Chosen language: it
data/wiki_dumps/itwiki-latest-pages-articles.xml.bz2 already exists. Skipping download.
data/wiki_extr/it already exists. Skipping extraction.
/src/fastai/.venv/bin/python: No module named multifit.create_wikitext
/src/fastai/.venv/bin/python: No module named multifit.postprocess_wikitext
/src/fastai/.venv/bin/python: No module named multifit.postprocess_wikitext