Fix link from generated configuration file to documentation (#11678)

Co-authored-by: reivilibre <olivier@librepush.net>
Co-authored-by: reivilibre <oliverw@matrix.org>

changelog.d/11678.doc

@@ -0,0 +1 @@
+Fix the documentation link in newly-generated configuration files.

docs/sample_config.yaml

@@ -37,7 +37,7 @@
 
 # Server admins can expand Synapse's functionality with external modules.
 #
-# See https://matrix-org.github.io/synapse/latest/modules.html for more
+# See https://matrix-org.github.io/synapse/latest/modules/index.html for more
 # documentation on how to configure or create custom modules for Synapse.
 #
 modules:

synapse/config/modules.py

@@ -37,7 +37,7 @@ class ModulesConfig(Config):
 
             # Server admins can expand Synapse's functionality with external modules.
             #
-            # See https://matrix-org.github.io/synapse/latest/modules.html for more
+            # See https://matrix-org.github.io/synapse/latest/modules/index.html for more
             # documentation on how to configure or create custom modules for Synapse.
             #
             modules: