message: Use placeholder instead of key expiration date
diff: @@ -6,7 +6,7 @@ Make sure you obtain the proper release signing key and verify the fingerprint w
  ```sh
  $ gpg --fingerprint "Bitcoin Core binary release signing key"
  -pub 4096R/36C2E964 2015-06-24 [expires: 2017-02-13]
  +pub 4096R/36C2E964 2015-06-24 [expires: YYYY-MM-DD]
  Key fingerprint = 01EA 5486 DE18 A882 D4C2 6845 90C8 019E 36C2 E964
  uid Wladimir J. van der Laan (Bitcoin Core binary release signing key) <laanwj@gmail.com>
  ```
  