Bump Unix sockets intro version (#15924)

https://github.com/matrix-org/synapse/pull/15708 didn't quite make the cut for `1.88.0` this morning.

changelog.d/15924.feature

@@ -0,0 +1 @@
+Add Unix Socket support for HTTP Replication Listeners. Document and provide usage instructions for utilizing Unix sockets in Synapse. Contributed by Jason Little.

docs/usage/configuration/config_documentation.md

@@ -462,7 +462,7 @@ See the docs [request log format](../administration/request_log.md).
 * `additional_resources`: Only valid for an 'http' listener. A map of
    additional endpoints which should be loaded via dynamic modules.
 
-Unix socket support (_Added in Synapse 1.88.0_):
+Unix socket support (_Added in Synapse 1.89.0_):
 * `path`: A path and filename for a Unix socket. Make sure it is located in a
   directory with read and write permissions, and that it already exists (the directory
   will not be created). Defaults to `None`.