chore: update README

Update preview image.

README.md

@@ -1,7 +1,7 @@
 #Icarus
 
 ### The blog theme you may fall in love with, coming to Hexo. [Preview](http://ppoffice.github.io/hexo-theme-icarus/)
-![](http://ppoffice.github.io/hexo-theme-icarus/gallery/preview.jpg "")
+![](http://ppoffice.github.io/hexo-theme-icarus/gallery/preview.png "")
 
 #### [View Documentation](https://github.com/ppoffice/hexo-theme-icarus/wiki)
 :star: It is strongly recommended that you read the docs before using Icarus.