doc: Mac -> macOS in release notes template

bitcoin/bitcoin#26503 (comment)

Author: fanquake

doc/release-notes-empty-template.md

@@ -25,7 +25,7 @@ How to Upgrade
 
 If you are running an older version, shut it down. Wait until it has completely
 shut down (which might take a few minutes in some cases), then run the
-installer (on Windows) or just copy over `/Applications/Bitcoin-Qt` (on Mac)
+installer (on Windows) or just copy over `/Applications/Bitcoin-Qt` (on macOS)
 or `bitcoind`/`bitcoin-qt` (on Linux).
 
 Upgrading directly from a version of Bitcoin Core that has reached its EOL is