Fix marked checkbox with ambiguous Markdown link

[cixtor]

This pull-request fixes bug in the Markdown parser used by GitHub to translate [x] into HTML checkboxes, the way it is currently written makes the transpiler believe that the text in between parenthesis is actually part of a Markdown link with this syntax [title](href) which makes me believe that the parser is taking /\[x\][^(]+\([^)]+\)/ (notice the pattern in between the braces and the parenthesis).

So instead of this:

• (very dated) http://dondodge.typepad.com/the_next_big_thing/2010/09/how-to-get-a-job-at-google-interview-questions-hiring-process.html

... it is rendering this:

• [x](very dated) http://dondodge.typepad.com/the_next_big_thing/2010/09/how-to-get-a-job-at-google-interview-questions-hiring-process.html