Remove remaining hard-coded reference to "aws-vault"

[solvingj]

https://github.com/99designs/keyring/blob/master/wincred.go#L78

It looks like this is a remnant of the time before this library was a separate project from aws-vault application.

Of note, based on cursory testing, it seems that the result of this is that windows credentials will always show up under aws-vault: service name. When I set ServiceName variable on the configuration myself to xyz, the credential appears in Windows Credential manager as aws-vault:xyz:<my-key>. I am new to the project, but it looks like simply removing "aws-vault:" + from this line would resolve the issue.

Also, I checked to see if this would break aws-vault project, and I don't think it will. I actually think it fixes a cosmetic bug there, but I don't use aws-vault so I can't be sure. Based on this line and my own test, I believe that currently, aws-vault creds are probably showing up in Windows Credential Manager as aws-vault:aws-vault:<some_cred>

https://github.com/99designs/aws-vault/blob/master/cli/global.go#L83

Perhaps this will be trivial to prove and fix for someone on the project.