rename theme unit lessons

docs/index.md

@@ -11,10 +11,10 @@ hero:
 
 ## [Themes](./howtostudykorean/theme/index.md)
 
-- [School](./howtostudykorean/theme/school.md)
-- [Transportation](./howtostudykorean/theme/transportation.md)
-- [Eating](./howtostudykorean/theme/eating.md)
-- [Shopping](./howtostudykorean/theme/shopping.md)
+- [School](./howtostudykorean/theme/01-school.md)
+- [Transportation](./howtostudykorean/theme/02-transportation.md)
+- [Eating](./howtostudykorean/theme/03-eating.md)
+- [Shopping](./howtostudykorean/theme/04-shopping.md)
 
 ## Unit 1