Update 404.html

A PROPER sad smiley. 

I think and believe that the smiley has to look familiar to the user who ends up loading 404.html into his browser. The 'sad' smiley must look familiar to what they see in their mobile devices. Hence just looking at the sad smiley must make them realize that something is wrong in the page without going into lot of details. Earlier I had proposed a similar change but that was not effective as I did not fore see it before uploading the whole thing, but now I feel this change is effective and also the increase in file size is very low and ignorable. Hope this makes way into every future release of Boilerplate.