Fix typo in faster join docs (#15812)

Fixes #15756

changelog.d/15812.doc

+Fix typo in MSC number in faster remote room join architecture doc.

docs/development/synapse_architecture/faster_joins.md

@@ -6,7 +6,7 @@ This is a work-in-progress set of notes with two goals:
 
 See also [MSC3902](https://github.com/matrix-org/matrix-spec-proposals/pull/3902).
 
-The key idea is described by [MSC706](https://github.com/matrix-org/matrix-spec-proposals/pull/3902). This allows servers to
+The key idea is described by [MSC3706](https://github.com/matrix-org/matrix-spec-proposals/pull/3706). This allows servers to
 request a lightweight response to the federation `/send_join` endpoint.
 This is called a **faster join**, also known as a **partial join**. In these
 notes we'll usually use the word "partial" as it matches the database schema.