fix tag variables (#1056)

docs/documentation/elements/tag.html

@@ -5,9 +5,11 @@ doc-tab: elements
 doc-subtab: tag
 variables:
 - name: $tag-background-color
-  value: background
+  value: $background
 - name: $tag-color
-  value: text
+  value: $text
+- name: $tag-radius
+  value: $radius
 ---
 
 {% capture tag %}