docs: ABI: correct QEMU fw_cfg spec path

A few weeks ago, QEMU switched docs/specs/fw_cfg.txt to be
docs/specs/fw_cfg.rst, so update the reference in the kernel docs to
reflect this. Also add a link to the online documentation to make it
easier to find.

Cc: Simon Sapin <[e-mail]>
Cc: Gabriel Somlo <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg

···
 		configuration data to the guest userspace.
 
 		The authoritative guest-side hardware interface documentation
-		to the fw_cfg device can be found in "docs/specs/fw_cfg.txt"
-		in the QEMU source tree.
+		to the fw_cfg device can be found in "docs/specs/fw_cfg.rst"
+		in the QEMU source tree, or online at:
+		https://qemu-project.gitlab.io/qemu/specs/fw_cfg.html
 
 		**SysFS fw_cfg Interface**