Document issue: sign command does not exist in `otp` / `load` section

[lff0305]

Inside README.md otp / load section there is an example

picotool sign hello_world.elf hello_world.signed.elf private.pem otp.json
......

But actually sign command cannot be found

ERROR: Unknown command: sign

[lurch]

How did you build and/or install picotool? What version number does it report?

EDIT: Hmmm, picotool sign actually appears 3 times in https://github.com/raspberrypi/picotool/blob/develop/README.md - were these supposed to be picotool seal @will-v-pi ?

[will-v-pi]

sign was replaced with seal --sign during development, so these are probably references that got missed

[lurch]

[lurch]

I guess this is partially addressed by #220 too.