minor #43903 Fix more generic types (derrabus)

This PR was merged into the 5.4 branch.

Discussion
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Fix more generic types

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | N/A
| License       | MIT
| Doc PR        | N/A

Some changes I wanted to push to #42471, but then it got merged already. 😇

Commits
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c54d61e9af Fix more generic types

Helper/HelperSet.php

@@ -23,9 +23,7 @@
  */
 class HelperSet implements \IteratorAggregate
 {
-    /**
-     * @var Helper[]
-     */
+    /** @var array<string, Helper> */
     private $helpers = [];
     private $command;