Update ReadMe.md

Author: zhihuanli

ReadMe.md

@@ -93,6 +93,6 @@
 - [作业 6.1](https://zhihuanli.github.io/Experimental-Data-Analysis-Course/chapt6/coursework6.1.html)
 
 ## chapter 7. 核反应数据处理(不变质量谱)
- - 7.1 [Relativistic Kinematics](https://zhihuanli.github.io/Experimental-Data-Analysis-Course/chapt7/relativistic kinematics.html)
+ - 7.1 [Relativistic Kinematics](https://zhihuanli.github.io/Experimental-Data-Analysis-Course/chapt7/relativistic_kinematics.html)
  - 7.2 [Invariant mass and missing mass](https://zhihuanli.github.io/Experimental-Data-Analysis-Course/chapt7/invariant_mass.html)
  - 7.3