Backport 0.21.0 release note updates

see: bitcoin/bitcoin@30e664d

Author: azuchi

_releases/0.21.0.md

@@ -125,7 +125,7 @@ P2P and network changes
 - This release adds support for serving
   [BIP157](https://github.com/bitcoin/bips/blob/master/bip-0157.mediawiki) compact
   filters to peers on the network when enabled using
-  `-blockfilterindex=1 -peercfilters=1`. (#16442)
+  `-blockfilterindex=1 -peerblockfilters=1`. (#16442)
 
 - This release adds support for signets
   ([BIP325](https://github.com/bitcoin/bips/blob/master/bip-0325.mediawiki)) in