Fix typo causing a task to be repeated in debug. Addresses BUKKIT-2549

An internal method for making the debug output for CraftScheduler's
async tasks was erroneously using the 'this' reference when the loop
should be referencing the current task.

src/main/java/org/bukkit/craftbukkit/scheduler/CraftAsyncDebugger.java

@@ -30,7 +30,7 @@ class CraftAsyncDebugger {
 
     StringBuilder debugTo(final StringBuilder string) {
         for (CraftAsyncDebugger next = this; next != null; next = next.next) {
-            string.append(plugin.getDescription().getName()).append(':').append(clazz.getName()).append('@').append(expiry).append(',');
+            string.append(next.plugin.getDescription().getName()).append(':').append(next.clazz.getName()).append('@').append(next.expiry).append(',');
         }
         return string;
     }