Fix nil template warning and validation for id

layout:nil has been deprecated in favor of layout:null (https://github.com/jekyll/jekyll/issues/2712)

The first id tag is missing it's trailing slash to pass validation. Here is the error from validator.w3.org: "In addition, interoperability with the widest range of feed readers could be improved by implementing the following recommendation. Identifier "http://hyde.getpoole.com" is not in canonical form (the canonical form would be "http://hyde.getpoole.com/")