Update ch1.md

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 http://meta.stackexchange.com/questions/51404/markdown-handles-inline-bold-text-within-a-word-incorrectly
Merge pull request #175 from rlbisbe/master

Changed Markdown for HTML to get AST definition properly rendered