Fix contents in tutorials

- Correct the aspect ratio of the mockup image
- Delete code snippet space in docs

_posts/2019-08-08-text-and-typography.md

@@ -8,8 +8,8 @@ math: true
 mermaid: true
 image:
   src: /commons/devices-mockup.png
-  width: 850
-  height: 585
+  width: 800
+  height: 500
 ---
 
 This post is to show Markdown syntax rendering on [**Chirpy**](https://github.com/cotes2020/jekyll-theme-chirpy/fork), you can also use it as an example of writing. Now, let's start looking at text and typography.