Fix schema of m.receipt EDU

[zecakeh]

It was missing a nesting level for the user ID -> user read receipt map.

Preview: https://pr1636--matrix-spec-previews.netlify.app

[richvdh]

A few links:

• The corrected rendered version is at https://pr1636--matrix-spec-previews.netlify.app/server-server-api/#definition-mreceipt
• The incorrect version is at https://spec.matrix.org/v1.8/server-server-api/#definition-mreceipt. (Though the example is correct.)
• It seems to have been wrong since forever (it was introduced in e03bfbc)

[richvdh]

I think we should document what the key of the object is (the user id), much as we do for the content object above it:

matrix-spec/data/api/server-server/definitions/event-schemas/m.receipt.yaml

Lines 34 to 35 in 9c995df

	Receipts for a particular room. The string key is the room ID for
	which the receipts under it belong.

[zecakeh]

I documented it

[richvdh]

thanks!