Fix spelling error.

--- proftpd-mod-prometheus.orig/mod_prometheus.c
+++ proftpd-mod-prometheus/mod_prometheus.c
@@ -1450,7 +1450,7 @@
 
   metric = prom_metric_create(prometheus_pool, "directory_list", dbh);
   prom_metric_add_counter(metric, "total",
-    "Number of succesful directory listings");
+    "Number of successful directory listings");
   prom_metric_add_gauge(metric, "count", "Current count of directory listings");
   res = prom_registry_add_metric(prometheus_registry, metric);
   if (res < 0) {
