tests: fix inaccurate comment in bootif test

The link was to testiso code, which isn't the code being excercised
here. Let's just update the comment to say it's the default one QEMU
assigns.

tests/kola/networking/no-default-initramfs-net-propagation/bootif

@@ -3,8 +3,8 @@
 ##   # appendFirstbootKernelArgs is only supported on qemu
 ##   platforms: qemu
 ##   # Append BOOTIF kernel argument so we can test how nm-initrd-generator
-##   # and the coreos-teardown-initramfs interact. The MAC address is from:
-##   # https://github.com/coreos/coreos-assembler/blob/d5f1623aad6d133b2c7c00e784c04ab6828450c1/mantle/platform/metal.go#L468
+##   # and the coreos-teardown-initramfs interact. The MAC address is the
+##   # default one QEMU assigns.
 ##   # Add rd.neednet=1 so we can force networking to be brought up on
 ##   # qemu to test that doing so doesn't materially change things.
 ##   appendFirstbootKernelArgs: "BOOTIF=52:54:00:12:34:56 rd.neednet=1"