add sentinel webflux example and modify sentinel web example

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

@@ -31,6 +31,11 @@
             <artifactId>spring-boot-starter-actuator</artifactId>
         </dependency>
 
+        <dependency>
+            <groupId>com.alibaba.csp</groupId>
+            <artifactId>sentinel-web-servlet</artifactId>
+        </dependency>
+
         <!--<dependency>-->
         <!--<groupId>com.alibaba.csp</groupId>-->
         <!--<artifactId>sentinel-datasource-nacos</artifactId>-->

spring-cloud-alibaba-examples/sentinel-example/sentinel-core-example/src/main/resources/flowrule.json

@@ -1,6 +1,6 @@
 [
   {
-    "resource": "resource",
+    "resource": "/hello",
     "controlBehavior": 0,
     "count": 1,
     "grade": 1,