docs: add scoped option in example file (#3747)

docs/examples/card/simple.vue

@@ -3,7 +3,7 @@
     <div v-for="o in 4" :key="o" class="text item">{{ 'List item ' + o }}</div>
   </el-card>
 </template>
-<style>
+<style scoped>
 .text {
   font-size: 14px;
 }

docs/examples/tooltip/basic.vue

@@ -108,7 +108,7 @@
   </div>
 </template>
 
-<style lang="scss">
+<style lang="scss" scoped>
 .box {
   width: 400px;