add dubbo demo in sentinel example

spring-cloud-alibaba-examples/sentinel-example/pom.xml

@@ -30,6 +30,12 @@
             <artifactId>spring-boot-starter-actuator</artifactId>
         </dependency>
 
+        <dependency>
+            <groupId>com.alibaba.boot</groupId>
+            <artifactId>dubbo-spring-boot-starter</artifactId>
+            <version>0.2.0</version>
+        </dependency>
+
     </dependencies>
 
     <build>