fix: markdown slots doesn't work (close: #1048)(#1054)

It's because we didn't read slotKey prop at Content component
This commit also updated the documentation.

Author: janvennemann

Diff for: packages/@vuepress/core/lib/app/components/Content.js

@@ -19,7 +19,7 @@ export default {
         class: [data.class, data.staticClass],
         style: data.style,
         props: {
-          slotKey: props.slot || 'default'
+          slotKey: props.slotKey || 'default'
         }
       })
     }

Diff for: packages/docs/docs/guide/markdown-slot.md

@@ -17,15 +17,15 @@ Markdown Slot is to solve this kind of problem.
 You can define a named markdown slot through the following markdown syntax:
 
 ``` md
-::: slot [$name]
+::: slot name
 
 :::
 ```
 
 Use the `Content` component to use the slot in the layout component:
 
 ``` vue
-<Content slot="$name"/> 
+<Content slot-key="name"/>
 ```
 
 ## Default Slot Content