Use tech site URL for image (#12)

Author: microbit-mark

spec/index.md

@@ -20,7 +20,7 @@ The goal of a micro:bit Universal Hex is to be able to create a single-file form
 
 ## Definitions and Rationale
 
-<img align="right" src="img/universal-hex.png" alt="universal hex">
+<img align="right" src="https://tech.microbit.org/software/spec-universal-hex/spec/img/universal-hex.png" alt="universal hex">
 
 Universal Hex is a superset of the Intel Hex file format to be able to contain binary code for multiple micro:bit boards or subsystems.