MINOR: Fix missing argument in kafka-features.sh upgrade doc (#19160)

Running `bin/kafka-features.sh upgrade --release-version 4.0` results in
the following error. This PR fixes the issue by adding the required
argument.

`kafka-features: error: one of the arguments --bootstrap-server
--bootstrap-controller is required.`

Reviewers: Colin P. McCabe <[email protected]>

Author: MahsaSeifikar

docs/upgrade.html

@@ -29,7 +29,7 @@ <h4><a id="upgrade_4_0_0" href="#upgrade_4_0_0">Upgrading to 4.0.0 from any vers
     </li>
     <li>Once the cluster's behavior and performance has been verified, finalize the upgrade by running
         <code>
-            bin/kafka-features.sh upgrade --release-version 4.0
+            bin/kafka-features.sh --bootstrap-server localhost:9092 upgrade --release-version 4.0
         </code>
     </li>
     <li>Note that cluster metadata downgrade is not supported in this version since it has metadata changes.