bpf, docs: Update ISA document title

* Use "Instruction Set Architecture (ISA)" instead of "Instruction Set
  Specification"
* Remove version number

As previously discussed on the mailing list at
https://mailarchive.ietf.org/arch/msg/bpf/SEpn3OL9TabNRn-4rDX9A6XVbjM/



Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20240208221449.12274-1-dthaler1968@gmail.com