docs: Update the location of the LF NDA program

The link to the Linux Foundation NDA program got broken in one of their
web-site thrashups; now that the information is back online, point to its
current location.  This should last until the next thrashup...

Reported-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>