Use Asciidoctor 1.5.0 new syntax backtick

Asciidoctor 1.5.0 is now using backtick for inline code.
Replaces ASCIIDoc by AsciiDoc to respect the case.
Merge pull request #862 from Mogztter/asciidoctor-1.5.0-syntax

Use Asciidoctor 1.5.0 new syntax backtick