Tutorial: Add missing entry in Step4 file list

The root's `CMakeLists.txt` is also one of the files the need editing in
Exercise 1, but was missing.

Help/guide/tutorial/In-Depth CMake Target Commands.rst

@@ -162,6 +162,7 @@ Helpful Resources
 Files to Edit
 -------------
 
+* ``CMakeLists.txt``
 * ``Tutorial/CMakeLists.txt``
 * ``MathFunctions/CMakeLists.txt``
 * ``MathFunctions/MathFunctions.cxx``