fix scaladoc cache bug. when use -doc-root-content

### steps

Define build.sbt that uses scaladoc `-doc-root-content` option.

```scala
scalacOptions in (Compile, doc) := Seq("-doc-root-content", "root.txt")
```

1. Run `doc` task from the shell.
2. Change the content of root.txt.
3. Run `doc` task again.

### problem

The `doc` does not reflect the change in root.txt.

### what this pull request changes

The cache is updated if the file specified by `-doc-root-content` changes.