README: `ownershipFile` requires a File, not a String

[Roberto Leinardi]

The README.md shows that the ownershipFile should be set using a String when instead it requires a File.

Actual:

ruler {
    ownershipFile.set("/path/to/ownership.yaml")
    defaultOwner.set("default-team") // unknown by default
}

Expected:

ruler {
    ownershipFile.set(rootProject.file("/path/to/ownership.yaml"))
    defaultOwner.set("default-team") // unknown by default
}