Update _bertopic.py

[nbalepur]

Adding functionality in topics_over_time to allow users to specify how many terms under each topic they want to see at each timestep t. Right now, the default value is 5, but there are use cases where the user may want to see more than 5

[MaartenGr]

Thank you for the PR. The difficulty with opening up the top_n_words_per_time variable here is that it then should be done for every other variation of BERTopic that generates top n words. This makes the combination with the original top_n_words a bit more complex and less intuitive especially when it needs to be implemented for many variations.