Modifying warning due to 404 extension (#202)

Thanks @djcaesar9114 👏

_config.yml

@@ -159,7 +159,7 @@ social:
 # 404 page #
 ############
 
-404-img: "/img/labtocat.png"
+err-404-img: "/img/labtocat.png"
 
 ##################
 # GPG on Keybase #

_layouts/error.html

@@ -18,7 +18,7 @@
 
           {{ page.content | markdownify }}
 
-          <img class="img-responsive center-block fourofour" src="{{site.baseurl}}{{ site.404-img }}" alt="">
+          <img class="img-responsive center-block fourofour" src="{{site.baseurl}}{{ site.err-404-img }}" alt="">
 
         </div>
       </div>

_posts/2015-06-25-initial-setup.md

@@ -68,7 +68,7 @@ title: "{ John Smith }"
 description: "Blog and website of John Smith, blogging mainly for tech. Opinions expressed are mine."
 keywords: "smith, jones, personal, jekyll, theme"
 favicon: "/img/favicon.ico"
-404-img: "/img/labtocat.png"
+err-404-img: "/img/labtocat.png"
 </code></pre>
 
 The values that you set, will be placed in the head section of every generated HTML page.