Update link to manual

Author: Wilfred

README.md

@@ -1,7 +1,7 @@
 <p align="center">
   <a href="#readme"><img src="img/logo.png" alt="it's difftastic!"/></a>
   <br>
-  <a href="http://difftastic.wilfred.me.uk/"><img src="https://img.shields.io/badge/manual-en-brightgreen?style=flat-square" alt="English manual"></a>
+  <a href="https://difftastic.wilfred.me.uk/introduction.html"><img src="https://img.shields.io/badge/manual-en-brightgreen?style=flat-square" alt="English manual"></a>
   <a href="https://difftastic.wilfred.me.uk/zh-CN/"><img src="https://img.shields.io/badge/manual-zh--CN-brightgreen?style=flat-square" alt="Chinese manual"></a>
   <a href="https://crates.io/crates/difftastic"><img src="https://img.shields.io/crates/v/difftastic.svg?style=flat-square" alt="crates.io"></a>
   <a href="https://codecov.io/gh/Wilfred/difftastic"><img src="https://img.shields.io/codecov/c/github/Wilfred/difftastic?style=flat-square&token=dZzAZtQT2S" alt="codecov.io"></a>