minor #15876 [Form] Fix comments (sebpacz)

This PR was merged into the 4.4 branch.

Discussion
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[Form] Fix comments

I found two comments with code that will throw an exception. The `getExtendedTypes` method should return an `iterable` value, while code in the comment returns a `string` value.

Commits
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c06ec56 [Form] Fix comments

Author: javiereguiluz

form/create_form_type_extension.rst

@@ -35,11 +35,11 @@ First, create the form type extension class extending from
     class ImageTypeExtension extends AbstractTypeExtension
     {
         /**
-         * Return the class of the type being extended.
+         * Returns an array of extended types.
          */
         public static function getExtendedTypes(): iterable
         {
-            // return FormType::class to modify (nearly) every field in the system
+            // return [FormType::class] to modify (nearly) every field in the system
             return [FileType::class];
         }
     }
@@ -145,7 +145,7 @@ For example::
     {
         public static function getExtendedTypes(): iterable
         {
-            // return FormType::class to modify (nearly) every field in the system
+            // return [FormType::class] to modify (nearly) every field in the system
             return [FileType::class];
         }