Modify section title according to Asciidoctor's html5 converter
Modify section title according to Asciidoctor's html5 converter

See:
* https://github.com/asciidoctor/asciidoctor/commit/bab7035f50dcd6d4d5d47b33842619ec9a951719
* https://github.com/asciidoctor/asciidoctor/commit/845322cade05f2b6676d23a09bee273a06c1e9a4

jirutka/asciidoctor-html5,jirutka/asciidoctor-html5