Fix dead link in spam checker warning (#12231)

changelog.d/12231.doc

+Fix the link to the module documentation in the legacy spam checker warning message.

synapse/config/spam_checker.py

@@ -25,8 +25,8 @@ logger = logging.getLogger(__name__)
 LEGACY_SPAM_CHECKER_WARNING = """
 This server is using a spam checker module that is implementing the deprecated spam
 checker interface. Please check with the module's maintainer to see if a new version
-supporting Synapse's generic modules system is available.
-For more information, please see https://matrix-org.github.io/synapse/latest/modules.html
+supporting Synapse's generic modules system is available. For more information, please
+see https://matrix-org.github.io/synapse/latest/modules/index.html
 ---------------------------------------------------------------------------------------"""