Update README.md

Author: elect86

README.md

@@ -21,7 +21,7 @@ Status:
 - [x] Chapter 4, Objects at Rest
 - [x] Chapter 5, Objects at Depth
 - [x] Chapter 6, Objects in Motion
-- [ ] Chapter 7, World in Motion
+- [x] Chapter 7, World in Motion
 - [ ] Chapter 8, Getting Oriented
 - [ ] Chapter 9, Lights On
 - [ ] Chapter 10, Plane Lights