README: use new miner threads flag instead of legacy minerthreads flag (#20165)

README.md

@@ -303,7 +303,7 @@ ones either). To start a `geth` instance for mining, run it with all your usual
 by:
 
 ```shell
-$ geth <usual-flags> --mine --minerthreads=1 --etherbase=0x0000000000000000000000000000000000000000
+$ geth <usual-flags> --mine --miner.threads=1 --etherbase=0x0000000000000000000000000000000000000000
 ```
 
 Which will start mining blocks and transactions on a single CPU thread, crediting all