fix publishing instruction on "Community Plugins"

## steps

1. follow the "Community Plugins" page to publish the first plugin on `repo.scala-sbt.org` after an admin creates an account.

## problem

```
[error] (*:publishSigned) java.io.IOException: Access to URL http://scalasbt.artifactoryonline.com/scalasbt/sbt-plugin-releases/com.example/sbt-foo/scala_2.10/sbt_0.13/0.1.0/docs/sbt-foo-javadoc.jar.asc was
 refused by the server: Unauthorized
```

This is because the current instruction page for 0.12.4 says to use repo.scala-sbt.org but use `"scalasbt.artifactoryonline.com"` for credential.

## what this changes

This backports the fix sbt/sbt@89d15ed from 0.13 branch.