Improved preprocessing instructions: delete "files" directories even if not empty

preprocessing/README.md

@@ -32,7 +32,7 @@
 
   $ find . -type d -iname files -execdir mv ./files/main.cpp . \; -print
 
-  $ find . -type d -iname files -delete -print
+  $ find . -type d -iname files -execdir rm -rf \{\} + -print
   ```
 
 * Now, each `<user>` directory should only have two files in it: `<user>/main.cpp` and `<user>/details.json`