fix duplicate maxLevel (docsifyjs#437)

Author: dalphyx

docs/zh-cn/configuration.md

@@ -40,7 +40,7 @@ window.$docsify = {
 };
 ```
 
-## max-level
+## maxLevel
 
 * 类型：`Number`
 * 默认值: `6`
@@ -53,19 +53,6 @@ window.$docsify = {
 };
 ```
 
-## maxLevel
-
-* Type: `Number`
-* Default: `6`
-
-Maximum Table of content level.
-
-```js
-window.$docsify = {
-  maxLevel: 4
-};
-```
-
 ## loadNavbar
 
 * 类型：`Boolean|String`