Tutorial: Clarify the expected change to target_include_directories

The expectation is to modify the existing `target_include_directories`,
not add add a new `target_include_directories` command.

Help/guide/tutorial/Adding a Library.rst

@@ -145,10 +145,10 @@ Next, the new library target is linked to the executable target using
 
   </details>
 
-Finally we need to specify the library's header file location. Modify
-:command:`target_include_directories` to add the ``MathFunctions`` subdirectory
-as an include directory so that the ``MathFunctions.h`` header file can be
-found.
+Finally we need to specify the library's header file location.
+Modify the existing :command:`target_include_directories` call
+to add the ``MathFunctions`` subdirectory as an include directory
+so that the ``MathFunctions.h`` header file can be found.
 
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