Fix check for new and unexpected roots

We track the "roots" in our grammar -- those productions that aren't
used in any other production.  We want to report when a new root
appears or when something that's expected to be a root no longer is
one.  However, we were reporting the latter case as the former instead
of reporting it separately as intended.  Let's fix that.

Author: traviscross

Diff for: mdbook-spec/src/grammar.rs

@@ -200,8 +200,8 @@ fn check_unexpected_roots(grammar: &Grammar, diag: &mut Diagnostics) {
     .into_iter()
     .collect();
     if set != expected {
-        let new: Vec<_> = set.symmetric_difference(&expected).collect();
-        let removed: Vec<_> = expected.symmetric_difference(&set).collect();
+        let new: Vec<_> = set.difference(&expected).collect();
+        let removed: Vec<_> = expected.difference(&set).collect();
         if !new.is_empty() {
             warn_or_err!(
                 diag,