fix unexport判断条件取反

git-svn-id: http://code.alibabatech.com/svn/dubbo/trunk@156 1a56cb94-b969-4eaa-88fa-be21384802f2

dubbo-config/src/main/java/com/alibaba/dubbo/config/ServiceConfig.java

@@ -250,7 +250,7 @@ public class ServiceConfig<T> extends AbstractServiceConfig {
     }
 
     public synchronized void unexport() {
-        if (exported) {
+        if (!exported) {
             throw new IllegalStateException("No exported!");
         }
         if (unexported) {