[Custom Attribute type] Corrected StorageSchema import

[mnocon]

Target: master, 4.6

The Custom Attribute type example is using the Ibexa\ProductCatalog\Local\Persistence\Legacy\Attribute\Boolean\StorageSchema class, but it's confusing - it should be using the Ibexa\ProductCatalog\Local\Persistence\Legacy\Attribute\Float\StorageSchema class.

Reasoning:

1. The PercentStorageDefinition is using Float (https://github.com/ibexa/documentation-developer/blob/master/code_samples/catalog/custom_attribute_type/src/Attribute/Percent/Storage/PercentStorageDefinition.php#L13) so it makes it consistent
2. The value type is float (https://github.com/ibexa/documentation-developer/blob/master/code_samples/catalog/custom_attribute_type/src/Attribute/Percent/Storage/PercentStorageDefinition.php#L20) and boolean looks out of context here.

It works by accident (because Ibexa\ProductCatalog\Local\Persistence\Legacy\Attribute\Boolean\StorageSchema::COLUMN_VALUE and Ibexa\ProductCatalog\Local\Persistence\Legacy\Attribute\Float\StorageSchema::COLUMN_VALUE have the same value - value), but it's better to make it consistent.

See:

• https://github.com/ibexa/product-catalog/blob/main/src/lib/Local/Persistence/Legacy/Attribute/Float/StorageSchema.php#L16
• https://github.com/ibexa/product-catalog/blob/main/src/lib/Local/Persistence/Legacy/Attribute/Boolean/StorageSchema.php#L16

Preview:
https://ez-systems-developer-documentation--2474.com.readthedocs.build/en/2474/pim/create_custom_attribute_type/#storage-converter