CSharpUtilities: Fix cross-reference in documentation

The documentation fix commit 04a11f16ba (CSharpUtilities: Fix
documentation, 2017-03-15, v3.8.0-rc3~17^2) introduced a cross-reference
to the function being documented.  Fix it.

Issue: #16711

Modules/CSharpUtilities.cmake

@@ -64,7 +64,7 @@ Main functions provided by the module
   Sets source file properties of ``.Designer.cs`` files depending on
   sibling filenames. Use this, if your CSharp target does **not**
   use Windows Forms (for Windows Forms use
-  :command:`csharp_set_designer_cs_properties` instead)::
+  :command:`csharp_set_windows_forms_properties` instead)::
 
     csharp_set_designer_cs_properties([<file1> [<file2> [...]]])