Small fix to the Documentation

[spundun]

I created a gist, which in hindsight was an overkill. https://gist.github.com/spundun/8d43cad2b04f7a96246e

The fix is for the page http://cjheath.github.io/treetop/pitfalls_and_advanced_techniques.html
In the section Matching Nested Structures With Non-Unique Delimeters, simply change the second rule to

rule em
  '*' (strong / !'*' . / '\*')+ '*'
end

instead of

rule em
  '**' (strong / !'*' . / '\*')+ '**'    
end

[Clifford Heath]

Thanks. I'll process this when I'm back at work mid next week.

[spundun]

Also consider mentioning the "first rule is the root rule" rule somewhere, as we discussed in #19

[Clifford Heath]

I processed this in Nov 2014 but didn't close the PR.