Upgrade Dubbo version in sentinel-demo-dubbo (#436)

sentinel-demo/sentinel-demo-dubbo/pom.xml

@@ -20,7 +20,7 @@
         <dependency>
             <groupId>com.alibaba</groupId>
             <artifactId>dubbo</artifactId>
-            <version>2.6.2</version>
+            <version>2.6.5</version>
         </dependency>
         <dependency>
             <groupId>com.alibaba.csp</groupId>