Fix piv-tool generate-key command in TOKENS doc

[cosign/TOKENS\.md at main · sigstore/cosign](https://github.com/sigstore/cosign/blob/main/TOKENS.md) says:

... *run the* `cosign generate-key` *command* ...

Which yields this error:

```
$ cosign generate-key
Error: unknown command "generate-key" for "cosign"
```

The documentation is simply missing the `piv-tool` sub-command.

Resolves: #1849

Signed-off-by: Neal McBurnett <neal@mcburnett.org>

TOKENS.md

@@ -18,7 +18,7 @@ We recommend using an application provided by your hardware vendor to manage key
 
 ### Setup
 
-To get started, insert a key to your computer and run the `cosign generate-key` command.
+To get started, insert a key to your computer and run the `cosign piv-tool generate-key` command.
 We recommend using the `--random-management-key=true` flag.
 
 This command generates a cryptographically-random management key and configures the device to use it.