Change JSON filename/miner ID on local lotus `add-miner` command

As per step 11 in this [local network doc](https://lotus.filecoin.io/lotus/developers/local-network/).

Running this command:

```
./lotus-seed pre-seal --sector-size 2KiB --num-sectors 2
```

Generate the following output:

```
INFO preseal seed/seed.go:228 Writing preseal manifest to /Users/pancy/.genesis-sectors/pre-seal-f01000.json
```

Note that the miner ID prefix has changed from `t` to `f` and the file written to is now `pre-seal-f01000.json`. Therefore, the file name should be corrected in the command in step 14 from ` ~/.genesis-sectors/pre-seal-t01000.json` to `~/.genesis-sectors/pre-seal-f01000.json`.