docs: fix quotes (#1905)

Author: a81n9

packages/varlet-ui/src/snackbar/docs/en-US.md

@@ -141,7 +141,7 @@ import { Snackbar } from '@varlet/ui'
 </script>
 
 <template>
-  <var-button type="warning" block @click="Snackbar("Hello, I'm a snackbar")">Basic Usage</var-button>
+  <var-button type="warning" block @click="Snackbar('Hello, I'm a snackbar')">Basic Usage</var-button>
 </template>
 ```
 

packages/varlet-ui/src/snackbar/docs/zh-CN.md

@@ -139,7 +139,7 @@ import { Snackbar } from '@varlet/ui'
 </script>
 
 <template>
-  <var-button type="warning" block @click="Snackbar("这是一个消息条！！")">基本使用</var-button>
+  <var-button type="warning" block @click="Snackbar('这是一个消息条！！')">基本使用</var-button>
 </template>
 ```