ExternalSpecsWriter currently requires an exact match for titles.

Given recent usage of `@spec` has shown use of long titles that
generally require line-wrapping, we should soften the check to
ignore whitespace differences, perhaps by normalizing it in
`checkUniqueItems`.

OLD:
            if (ii.getDocTree() instanceof SpecTree st) {
                String url = st.getURL().toString();
                String title = st.getTitle().toString();

NEW:
                String title = st.getTitle().toString().replaceAll("\\s+", " ");