Help/guide: Fix wrong header file reference

Step 2 references mysqrt.h instead of MathFunctions.h.
MathFunctions/mysqrt.h doesn't exist until Step 9.

Help/guide/tutorial/Adding a Library.rst

@@ -23,8 +23,9 @@ directory:
 To make use of the new library we will add an :command:`add_subdirectory`
 call in the top-level ``CMakeLists.txt`` file so that the library will get
 built. We add the new library to the executable, and add ``MathFunctions`` as
-an include directory so that the ``mysqrt.h`` header file can be found. The
-last few lines of the top-level ``CMakeLists.txt`` file should now look like:
+an include directory so that the ``MathFunctions.h`` header file can be found.
+The last few lines of the top-level ``CMakeLists.txt`` file should now look
+like:
 
 .. code-block:: cmake
         :caption: CMakeLists.txt