Changed Markdown for HTML to get AST definition properly rendered

Converted the Abstract Syntax Tree definition to use HTML tags instead of Markdown, looks like github's markdown have the same issue than Stack Overflow's

- **A**bstract: A is not bold (Markdown without space)
- **A** bstract: A is bold (Markdown with space)
- <b>A</b>bstract: A is bold (HTML without space)

http://meta.stackexchange.com/questions/51404/markdown-handles-inline-bold-text-within-a-word-incorrectly