fix(documentation): wrong import derived variables (#1726)

remove extension from derived variables import

docs/documentation/overview/customize.html

@@ -37,7 +37,7 @@ $github: #333;
 $github-invert: findColorInvert($github);
 
 // 5. Add new color variables to the color map.
-@import "../sass/utilities/derived-variables.sass";
+@import "../sass/utilities/derived-variables";
 $addColors: (
   "twitter":($twitter, $twitter-invert),
   "linkedin": ($linkedin, $linkedin-invert),