Update link to extensible events MSC

Signed-off-by: Nicolas Werner <[email protected]>

Author: deepbluev7

specification/modules/instant_messaging.rst

@@ -121,7 +121,7 @@ the tag and its contents and therefore may wish to exclude the tag entirely.
 
 .. Note::
    A future iteration of the specification will support more powerful and extensible
-   message formatting options, such as the proposal `MSC1225 <https://github.com/matrix-org/matrix-doc/issues/1225>`_.
+   message formatting options, such as the proposal `MSC1767 <https://github.com/matrix-org/matrix-doc/pull/1767>`_.
 
 {{msgtype_events}}