add in {% block main %} to index.liquid

http://doc.locomotivecms.com/making-blog/2-2-adding-pages inherits index.liquid and modifies the `block 'main'`. However, for me this didn't work b/c there was no `block 'main'` specified on the index.liquid template. Made this modification and then it worked. Short PR instead of an issue, to do with as you will :)