update(docs): update the cmd of python setup serve in py2 and py3. (#1473)

add py3 cmd lines.

Author: Koooooo-7

docs/quickstart.md

@@ -83,9 +83,12 @@ If you prefer to lock docsify to a specific version, specify the full version af
 
 If you installed python on your system, you can easily use it to run a static server to preview your site.
 
-```bash
+```python2
 cd docs && python -m SimpleHTTPServer 3000
 ```
+```python3
+cd docs && python -m http.server 3000
+```
 
 ## Loading dialog