s/KEBASE_PAPERKEY/KEYBASE_PAPERKEY/

This misleads users who attempt to follow the `--help` advice:

https://github.com/keybase/client/blob/08da17439d3cf9c6e03603f1126fca04ebf65799/go/service/main.go#L1242

```
to enable this option. We recommend passing the paper key via standard input, or
the KEBASE_PAPERKEY= environment variable, but this subcommand will dangerously accept
the paper key via command line option.`,
```

I think it makes sense to fix this alongside a nearby edit, tho I'm unsure how to identify those pull requests. Please let me know if you think I should mention the issue elsewhere or just rephrase it as a pull request.

Open go/service/main.go around line 1242 and inspect the --help text for the paper-key option. Correct the KEBASE_PAPERKEY typo to KEYBASE_PAPERKEY, then run the relevant command's help output to confirm the displayed environment variable name is accurate.

Written by the indexing model from the issue text.