Help: replace deprecated $<CONFIGURATION> generator expression

The `$<CONFIGURATION>` genex has been deprecated since CMake 3.0.
Use `$<CONFIG>` instead in the `add_test` example.

Help/command/add_test.rst

@@ -57,7 +57,7 @@ Example usage:
 .. code-block:: cmake
 
   add_test(NAME mytest
-           COMMAND testDriver --config $<CONFIGURATION>
+           COMMAND testDriver --config $<CONFIG>
                               --exe $<TARGET_FILE:myexe>)
 
 This creates a test ``mytest`` whose command runs a ``testDriver`` tool