Update image source for bulletproofs logo

Author: hdevalence

README.md

@@ -2,7 +2,7 @@
 
 <img
  width="100%"
- src="https://user-images.githubusercontent.com/698/46373713-9cc40280-c643-11e8-9bfe-2b0586e40369.png"
+ src="https://doc.dalek.rs/assets/bulletproofs-rangeproof.png"
 />
 
 The fastest [Bulletproofs][bp_website] implementation ever, featuring