fix(slow_subs): fix unit of time of the expire_interval default value

apps/emqx_slow_subs/etc/emqx_slow_subs.conf

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