fix single page menu urls, update example sites #151

exampleSite/content/about-subpage2.md

@@ -0,0 +1,24 @@
+---
+title: "Second About subpage"
+description: "Second About subpage"
+date: "2019-02-28"
+author: "Hugo Authors"
+slug: /about/subpage2
+menu:
+  about:
+    identifier: about-subpage-s2
+    parent: about
+    name: About Subpage
+    title: About Subpage
+    url: /about/subpage/
+    weight: 1
+  subpage:
+    identifier: about-subpage2-s2
+    parent: about
+    name: Second About subpage
+    title: Second About subpage
+    url: /about/subpage2/
+    weight: 10
+---
+
+Second About subpage