Simplify AggregationStrategy sample

[glebfox]

Environment

• Documentation version: 7.2
• Part of the documentation: Manual
• Existing page: https://doc.cuba-platform.com/manual-latest/gui_Table.html#gui_Table_aggregatable

Description of the bug or enhancement

See: forum

Doc sample code shows how to create a custom AggregationStrategy for a o-t-m field which is unclear for a user (see forum thread for proof) and in fact a rare case. A more simple sample with an entity description would be more convenient.