discv4.md: update text about node ordering in bucket

[ZHappiness0427]

I am confused。According to the document， the entries are sorted by time last seen — least-recently seen node at the head, most-recently seen at the tail. why is the new node added to the front of the bucket

[fjl]

Oh, good catch! The bucket order of nodes actually does not matter in discv4, but it's still good to fix this to make the spec internally consistent.

[weipin]

Wondering if the fix could also be applied to v5 (5.1 and 5.2) as well. https://github.com/ethereum/devp2p/blob/master/discv5/discv5-theory.md#node-table

Edited from my phone