fix broken link to word segmentation from mwe.md

[dan-zeman]

The mwe page is part of archived proposal from the time when UD v2 was being prepared. I think it actually did not point to the word_segmentation page that time, as there was another page within the same set of proposal, https://universaldependencies.org/v2/segmentation.html

[konstantinschulz]

Good point, thanks for fixing it!