package/spidev_test: update the upstream URL in Config.in

Update the upstream URL in the help text in Config.in. This
addresses the 'Invalid(404)' URL status in the package stats
web page output.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/spidev_test/Config.in

@@ -14,4 +14,4 @@ config BR2_PACKAGE_SPIDEV_TEST
 	  This means you won't have quad-pumped SPI support if your
 	  toolchain is too old.
 
-	  https://www.kernel.org/doc/Documentation/spi/spidev_test.c
+	  https://www.kernel.org/doc/html/latest/spi/index.html