fix edit link (#1261)

* fix edit link

* fix another one

Author: chenxsan

packages/gatsby-theme-mdx/src/components/sidebar.mdx

@@ -1,5 +1,5 @@
 - [Introduction](/)
-- [Getting Started](/getting-started)
+- [Getting Started](/getting-started/)
   - [Next.js](/getting-started/next)
   - [Gatsby](/getting-started/gatsby)
   - [Create React App](/getting-started/create-react-app)
@@ -9,7 +9,7 @@
   - [Zero](/getting-started/zero)
 - [Table of components](/table-of-components)
 - [Playground <sup style={{ color: '#444', textTransform: 'uppercase' }}>_alpha_</sup>](/playground)
-- [Guides](/guides)
+- [Guides](/guides/)
   - [Syntax highlighting](/guides/syntax-highlighting)
   - [Live code editor](/guides/live-code)
   - [Math blocks](/guides/math-blocks)
@@ -19,7 +19,7 @@
   - [Wrapper customization](/guides/wrapper-customization)
   - [Render MDX to the terminal](/guides/terminal)
   - [Vue <sup style={{ color: '#444', textTransform: 'uppercase' }}>_alpha_</sup>](/guides/vue)
-- [Advanced](/advanced)
+- [Advanced](/advanced/)
   - [API](/advanced/api)
   - [Runtime](/advanced/runtime)
   - [AST](/advanced/ast)
@@ -31,7 +31,7 @@
 - [Contributing](/contributing)
 - [Projects](/projects)
 - [Editors](/editors)
-- [Blog](/blog)
+- [Blog](/blog/)
 - [About](/about)
 - [Migrating from v0 to v1](/migrating/v1)
 - [V0](https://v0.mdxjs.com)