[fix] the exercise the set-up refers to is now called the basics exercise

exercises/exercise-grids/README.md

@@ -2,7 +2,7 @@
 <br>
 ## Problem set-up
 
-Here we will expand on what we've covered in the mainfile exercise, and the problem set up will remain the same.
+Here we will expand on what we've covered in the basics exercise, and the problem set up will remain the same.
 
 ## Preparing the exercise