Fix typos in customize.html (#1409)

docs/documentation/overview/customize.html

@@ -8,7 +8,7 @@ doc-subtab: customize
 {% capture sass %}
 // 1. Import the initial variables
 @import "../sass/utilities/initial-variables"
-@import "../sass/utilities/function";
+@import "../sass/utilities/functions";
 @import "../sass/utilities/derived-variables.sass";
 
 // 2. Set your own initial variables