fix(emqx_slow_subs): fix default expire value

apps/emqx_slow_subs/etc/emqx_slow_subs.conf

@@ -12,8 +12,8 @@ emqx_slow_subs {
 
     ## The eviction time of the record, which in the statistics record table
     ##
-    ## Default: 5m
-    expire_interval = 5m
+    ## Default: 300ms
+    expire_interval = 300ms
 
     ## The maximum number of records in the slow subscription statistics record table
     ##