[cleanup] path to files

exercises/exercise-mainfile/README.md

@@ -21,7 +21,7 @@ In the beginning there is a uniform pressure of $`1\cdot 10^5 Pa`$ in the whole
 
 ## Preparing the exercise
 
-* Navigate to the directory `dumux/tutorial/ex1`
+* Navigate to the directory `dumux-course/exercises/exercise-mainfile`
 
 <br><br>
 ### Task 1: Getting familiar with the code
@@ -149,7 +149,7 @@ timeLoop->finalize(leafGridView.comm());
 * Change to the build-directory
 
 ```bash
-cd ../../build-cmake/tutorial/ex1
+cd ../../build-cmake/exercises/exercise-mainfile
 ```
 
 * Compile all three executables `exercise1_1p_a` and `exercise1_1p_b` and `exercise1_1p_c`