Tutorial: Fix Step 5 solution after update to Step 6

Since commit 6e58a01f4e (Tutorial: Update step 6 style, 2022-10-06) the
`literalinclude` included an extra comment from step 6 instructions.
Fix the include bounds to remove the extra text.

Help/guide/tutorial/Installing and Testing.rst

@@ -145,7 +145,7 @@ are similar. To the end of the top-level ``CMakeLists.txt`` we add:
   :name: TODO 3,4: CMakeLists.txt-install-TARGETS
   :language: cmake
   :start-after: # add the install targets
-  :end-before: # enable testing
+  :end-before: # TODO 1: Replace enable_testing() with include(CTest)
 
 .. raw:: html