Improved docstring for src/dendropy/datamodel/treemodel/_tree.py/reroot_at_edge

[terrycojones]

The current docstring says

``length1`` and ``length2`` will be assigned to the new (sub-)edge leading
        to the old parent of the original edge, while ``length2`` will be
        assigned to the old child of the original edge.

which is clearly(?) wrong, since length2 is mentioned twice.

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[mmore500]

Good catch! Thanks for the fix.