Usage of non-PlottableHistogram attributes `edges` in to_writeable

[APN-Pucky]

Usage of non-PlottableHistogram attribute axis.edges:

uproot5/src/uproot/writing/identify.py

Line 336 in 56098c5

fXmin=axis.edges[0],

I think this should be axis[0][0] and axis[-1][1] following the protocol.

Context and full error log: https://github.com/APN-Pucky/babyyoda/blob/support_interface_matrix/examples/interface/uproot/histo1d.ipynb