docs: fix mappings field type join example

[dancer1325]

Description

• Fix example wrongly indicated
  • Reason: 🧠example goal is to index documents on different parents, but ONLY 1 is specified🧠

Version

• ALL

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[kolchfa-aws]

Thanks, @dancer1325. In the current documentation, no parent 2 exists in the example, so changing parent 1 to parent 2 will lead to errors unless we add parent 2. We have 2 options for the fix here:

1. Create a complete example with two parents and two children.
2. Change the comment from "Index two child documents, one for each parent:" to "Index two child documents for the same parent:".