improve spelling of minimum expiration hours check

Co-authored-by: Helmut K. C. Tessarek <tessarek@evermeet.cx>

src/config.rs

@@ -730,7 +730,7 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
     }
 
     if cfg.invitation_expiration_hours < 1 {
-        err!("`INVITATION_EXPIRATION_HOURS` has a minimum size of 1")
+        err!("`INVITATION_EXPIRATION_HOURS` has a minimum duration of 1 hour")
     }
 
     Ok(())