Update distribution.md

[saikumaru]

topic_distr provides topic scores for each doc, i.e. n- documents and m- topics for each of that doc

What does this PR do?

There is a documentation typo thats confusing and needs correction.

Eg

topic_distr, _ = topic_model.approximate_distribution(documents_array, window=4)

len(topic_distr)#n- number of documents
#output 4172
len(topic_distr[0])#m- number of topics
#output 60

Before submitting

• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes (if applicable)?
• Did you write any new necessary tests?

[MaartenGr]

Thanks for the PR, LGTM!