Fix broken link in doc (#7094)

The doc in
[dupsort.md](https://github.com/ledgerwatch/erigon/blob/devel/docs/programmers_guide/dupsort.md#erigon)
points to a non-existent `AbstractKV.md`

As best as I can tell, the `AbstractKV.md` was reworked and renamed in
commit

0bc61c0

then, this was later moved into erigon-lib. This commit simply repoints
the doc at this new location.

Co-authored-by: Jason Yellick <jason@enya.ai>

docs/programmers_guide/dupsort.md

@@ -153,7 +153,8 @@ Erigon
 This article target is to show tricky concepts on examples. Future
 reading [here](./db_walkthrough.MD#table-history-of-accounts)
 
-Erigon supports multiple typed cursors, see [AbstractKV.md](./../../ethdb/AbstractKV.md)
+Erigon supports multiple typed cursors, see the [KV
+Readme.md](https://github.com/ledgerwatch/erigon-lib/tree/main/kv)