Update the value of group_creation_prefix in sample config. (#8992)

Removes the trailing slash with causes issues with matrix.to/Element.

changelog.d/8992.doc

+Update the example value of `group_creation_prefix` in the sample configuration.

docs/sample_config.yaml

@@ -2366,7 +2366,7 @@ spam_checker:
 # If enabled, non server admins can only create groups with local parts
 # starting with this prefix
 #
-#group_creation_prefix: "unofficial/"
+#group_creation_prefix: "unofficial_"
 
 
 

synapse/config/groups.py

@@ -32,5 +32,5 @@ class GroupsConfig(Config):
         # If enabled, non server admins can only create groups with local parts
         # starting with this prefix
         #
-        #group_creation_prefix: "unofficial/"
+        #group_creation_prefix: "unofficial_"
         """