Use h2 in search results for accessibility

axe DevTools highlighted an issue with heading levels being skipped - this
changes the search results to use `h2` instead of `h3` to fix this issue

src/assets/js/search.js

@@ -85,7 +85,7 @@
       innerHtml +=
         '<li class="search-results__result">' +
           `<a href="${item.url}">` +
-            `<h3 class="search-results__result-title">${item.title}</h3>` +
+            `<h2 class="search-results__result-title">${item.title}</h2>` +
           '</a>' +
           (breadcrumbs.length ? `<div class='search-results__result-breadcrumbs'>${breadcrumbsString}</div>` : '') +
           `<p class="search-results__result-excerpt">${excerpt}</p>` +