Fix some f-strings.

mnpw/__init__.py

@@ -104,7 +104,7 @@ def main():  # noqa: MC0001
         check.run(args.timeout)
     except Exception as ex:  # pylint: disable=broad-except
         if args.errors and not args.dry_run:
-            notify(f"[HOSTNAME] Check {args.command} failed: {ex}.")
+            notify(f"[{HOSTNAME}] Check {args.command} failed: {ex}.")
         parser.error(str(ex))
 
     if check.ExitCode == nagios.NagiosCode.OK:
@@ -126,7 +126,7 @@ def main():  # noqa: MC0001
             f"Check status is invalid for a Nagios plugin ({check.ExitCode})."
         )
         if args.errors:
-            notify(f"[HOSTNAME] Check {args.command} status is invalid.")
+            notify(f"[{HOSTNAME}] Check {args.command} status is invalid.")
 
     if args.chain:
         print(check._stdout)  # pylint: disable=protected-access