Misunderstandable description in tracking notification

I just got an email saying that a new member is tracking me:

```
[redacted] is now tracking you on Keybase. Tracking on Keybase
is sort of like "following" on other sites, except it is cryptographically useful.
When you track someone, you sign their public key and proofs of identity, so you can tell
if they change. Consequently, as long as their identity doesn't change,
you can use someone's keybase username as an alias to their
public key. In other words, cool.
```

The phrase `sign their public key` is a bit misunderstanding since it could be interpreted as pgp signing as well which is not the case. More appropriate would be for example `sign the link between their public key and proofs of identity`.

Search the repository for the quoted tracking-notification text, since no file or test is named. Update the phrase so it describes signing the link between the public key and identity proofs, and verify the notification no longer suggests PGP signing.

Written by the indexing model from the issue text.