Modified the value of files in watch:src task in Gruntfile.js

Hi,

I tried generator-knockout with yo and added livereload by updating Gruntfile.js.

Then, I aware the following warning during "grunt preview".

Warning: An error occurred while processing a template (Cannot read property 'src' of undefined).
Warning: An error occurred while processing a template (Cannot read property 'src' of undefined).

I found the cause of this warning.
Could you review my commit for the issue? If it's OK, I appreciate if you apply my pull request.

Thank you.