Fix mistake setting ONLY_HOSTS for Reference.

This was a copy/paste mistake.

Author: ehuss

Diff for: src/bootstrap/src/core/build_steps/doc.rs

@@ -1237,7 +1237,6 @@ pub struct Reference {
 impl Step for Reference {
     type Output = ();
     const DEFAULT: bool = true;
-    const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         let builder = run.builder;