docs/kunit/start: Use in-tree 'kunit_defconfig'

docs/kunit/start: Use in-tree 'kunit_defconfig'

The kunit doc suggests users to get the default `kunitconfig` from an
external git tree.  However, the file is already located under the
`arch/um/configs/` of the kernel tree.  Because the local file is easier
to access and maintain, this commit updates the doc to use it.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>