minor #18388 Update value_resolver.rst (athib)

This PR was submitted for the 6.3 branch but it was merged into the 6.2 branch instead.

Discussion
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Update value_resolver.rst

Replace return type from `array` to `iterable` as it is defined in `ValueResolverInterface`

Commits
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d8365d2 Update value_resolver.rst

Author: javiereguiluz

controller/value_resolver.rst

@@ -242,7 +242,7 @@ values. That's why you must always return an array, even for single values::
 
     class BookingIdValueResolver implements ValueResolverInterface
     {
-        public function resolve(Request $request, ArgumentMetadata $argument): array
+        public function resolve(Request $request, ArgumentMetadata $argument): iterable
         {
             // get the argument type (e.g. BookingId)
             $argumentType = $argument->getType();