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>

Author: sj-aws

Documentation/dev-tools/kunit/start.rst

@@ -32,9 +32,8 @@ regular Kernel config, with the specific test targets as well.
 
 .. code-block:: bash
 
-	git clone -b master https://kunit.googlesource.com/kunitconfig $PATH_TO_KUNITCONFIG_REPO
 	cd $PATH_TO_LINUX_REPO
-	ln -s $PATH_TO_KUNIT_CONFIG_REPO/kunitconfig kunitconfig
+	cp arch/um/configs/kunit_defconfig kunitconfig
 
 You may want to add kunitconfig to your local gitignore.