Add border to cover image

Author: jeroenjanssens

README.md

@@ -3,7 +3,7 @@
 [![License: CC BY-ND 4.0](https://img.shields.io/badge/License-CC%20BY--ND%204.0-lightgrey.svg)](https://creativecommons.org/licenses/by-nd/4.0/)
 
 <a href="https://datascienceatthecommandline.com/">
-<img src="https://datascienceatthecommandline.com/2e/images/cover-small.png" width="224px" style=" /></a>
+<img src="https://datascienceatthecommandline.com/2e/images/cover-readme.png" width="224px" /></a>
 
 This repository contains the full text, data, and scripts used in the second edition of the book *Data Science at the Command Line* by Jeroen Janssens. The book is licensed under the Creative Commons Attribution-NoDerivatives 4.0 International License.