Try to fix image in readme

Author: janhohenheim

readme.md

@@ -1,6 +1,6 @@
 # Rust Standard Library Cookbook
 
-![Book cover](https://raw.githubusercontent.com/jnferner/rust-standard-library-cookbook/tree/master/github-resources/cover.png)
+![Book cover](https://github.com/jnferner/rust-standard-library-cookbook/blob/master/github-resources/cover.png)
 
 This is the official codebase for all code featured in Packt's [Rust Standard Library Cookbook](https://www.packtpub.com/application-development/rust-standard-library-cookbook), written by me and [Daniel Durante](https://github.com/durango).
 If you enjoy the code examples, please consider buying the book in order to profit from detailed explanations to every recipe.