updated website instruction text for DNS proof

Currently the instruction text on the website for a DNS proof reads

```
Please enter the following as a TXT entry in your DNS zone, exactly as it appears. If you need a "name" for your entry, give it @.
```

This will mislead users (like me) to place the TXT records at the APEX of the DNS zone, which can have operational side effects for other applications using TXT records at the same location.

Keybase supports a TXT records under the domain prefix "_keybase", the TXT record should be created there by default and the zone APEX should only be used as a fallback

I suggest to change the text for DNS proof on the website to

```
Please enter the following as a TXT entry in your DNS zone under the domain prefix "_keybase" (example: _keybase.example.com. IN TXT "...."), with the data part exactly as it appears. If you need a "name" for your entry, give it "_keybase". The base name of the zone, or "@" as the name, could be used as a fallback.
```

Locate the website text containing the quoted DNS-proof instruction and read the surrounding implementation or content entry. Update the wording to prefer the _keybase prefix and describe the zone apex or @ name only as a fallback; done means the website displays the requested guidance without changing the proof behavior.

Written by the indexing model from the issue text.