update collapse doc

examples/docs/en-US/collapse.md

@@ -59,6 +59,11 @@ You can expand multiple panels
       return {
         activeNames: ['1']
       };
+    },
+    methods: {
+      handleChange(val) {
+        console.log(val);
+      }
     }
   }
 </script>

examples/docs/es/collapse.md

@@ -59,6 +59,11 @@ Puede expandir varios paneles
       return {
         activeNames: ['1']
       };
+    },
+    methods: {
+      handleChange(val) {
+        console.log(val);
+      }
     }
   }
 </script>

examples/docs/zh-CN/collapse.md

@@ -58,6 +58,11 @@
       return {
         activeNames: ['1']
       };
+    },
+    methods: {
+      handleChange(val) {
+        console.log(val);
+      }
     }
   }
 </script>