lib: overflow: update reference to kunit-tool

Replace URL with an updated path to the full Documentation page
Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

lib/overflow_kunit.c

 // SPDX-License-Identifier: GPL-2.0 OR MIT
 /*
  * Test cases for arithmetic overflow checks. See:
- * https://www.kernel.org/doc/html/latest/dev-tools/kunit/kunit-tool.html#configuring-building-and-running-tests
+ * "Running tests with kunit_tool" at Documentation/dev-tools/kunit/start.rst
  *	./tools/testing/kunit/kunit.py run overflow [--raw_output]
  */
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt