docs: use echo statement with single quotes (#820)

Ivocin · ulivz · commit 45d079c5f2fc · 2018-09-16T01:29:09.000+08:00
to avoid ! error: `-bash: !": event not found`
diff --git a/docs/zh/guide/getting-started.md b/docs/zh/guide/getting-started.md
@@ -13,7 +13,7 @@
 yarn global add vuepress # 或者：npm install -g vuepress
 
 # 新建一个 markdown 文件
-echo "# Hello VuePress!" > README.md
+echo '# Hello VuePress!' > README.md
 
 # 开始写作
 vuepress dev .
