Update contest-rules.html.md

Found that the "summary" attribute in this file's meta information was the cause for Docpad to complain about not being able to (error is not copied from commandline, is what I remember off the top of my head) `unable to call replace method on undefined`.

I've put in a space (" ") so that the variable now holds and *empty string* as its value. Running the build with full content on my local machine was successful after this second edit.