Tutorial: Restore TODO 4 comment in step 7

Restore a comment added by commit f9ef15f3dc (Tutorial: Update step 7
style, 2022-10-07, v3.26.0-rc1~482^2) that was accidentally removed by
commit 8ddf32196c (Tutorial: Refactor MathFunctions code, 2023-02-10,
v3.27.0-rc1~441^2).

Help/guide/tutorial/Step7/MathFunctions/mysqrt.cxx

@@ -1,5 +1,6 @@
 #include "mysqrt.h"
 
+// TODO 4: include cmath
 #include <iostream>
 
 namespace mathfunctions {