MINOR: Fix help message for downgrade feature flag in kafka-features.sh (#20813)

The [original text](https://github.com/apache/kafka/pull/13459/files#diff-ce84e11f146fd8913d7e76fce74fc238d41466e689deb0599c8d7cb7cc6790ddL123-L125)
is `Downgrade`

Reviewers: Federico Valeri <[email protected]>, Ken Huang
 <[email protected]>, Chia-Ping Tsai <[email protected]>

Author: mingyen066

tools/src/main/java/org/apache/kafka/tools/FeatureCommand.java

@@ -153,7 +153,7 @@ private static void addUpgradeParser(Subparsers subparsers) {
 
     private static void addDowngradeParser(Subparsers subparsers) {
         Subparser downgradeParser = subparsers.addParser("downgrade")
-                .help("Upgrade one or more feature flags.");
+                .help("Downgrade one or more feature flags.");
         downgradeParser.addArgument("--metadata")
                 .help("DEPRECATED -- The level to which we should downgrade the metadata. For example, 3.3-IV0.")
                 .action(store());