Update customize.html (#1408)

For this customization to work, need to add more partial scss files as above
$addColors: (
  "twitter":($twitter, $twitter-invert), 
  "linkedin": ($linkedin, $linkedin-invert),
  "github": ($github, $github-invert)
)
$colors: map-merge($colors, $addColors)

Otherwise it failed in my sass build

docs/documentation/overview/customize.html

@@ -8,6 +8,8 @@ doc-subtab: customize
 {% capture sass %}
 // 1. Import the initial variables
 @import "../sass/utilities/initial-variables"
+@import "../sass/utilities/function";
+@import "../sass/utilities/derived-variables.sass";
 
 // 2. Set your own initial variables
 // Update blue