Documentation/hw-vuln: Unify filename specification in index

Most of the index.rst files in Documentation/ refer to other rst files
without their file extension in the name. Do that here too.

No functional changes.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20230809102700.29449-2-bp@alien8.de