MOBI Output: Fix regression in 0.8.30 that caused the use of hidden heading elements for the TOC to generate links in the wrong place. Fixes #907156 (Private bug)

src/calibre/ebooks/mobi/mobiml.py

@@ -325,6 +325,7 @@ class MobiMLizer(object):
                 elem.text = None
                 elem.set('id', id_)
                 elem.tail = tail
+                elem.tag = XHTML('a')
             else:
                 return
         tag = barename(elem.tag)