Documentation: kunit: Clean up some string casts in examples

As the type checking is no longer excessively strict, get rid of the
unsightly (char*) casts -- and comment discussing them -- from the KUnit
usage page.

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>