docs: fix distance_sort parameter description in tree_params.py

[Carbon14-48]

What

Fixed the distance_sort parameter description in malariagen_data/anoph/tree_params.py.

Why

The current description has two issues:

1. "if True" appears twice: "If True, for each node n, if True, the child..."
2. "distance between" is incomplete — it doesn't say between what.

Fix

• Removed the duplicated "if True"
• Clarified that the distance is between the node's direct descendants, consistent with scipy's dendrogram(distance_sort) documentation.

The count_sort description directly above was already correct and served as a reference for the fix.