Update modular.html

Added missing `.sass` extensions.

docs/documentation/overview/modular.html

@@ -11,8 +11,8 @@ breadcrumb:
 ---
 
 {% capture import %}
-@import "bulma/sass/utilities/_all"
-@import "bulma/sass/grid/columns"
+@import "bulma/sass/utilities/_all.sass"
+@import "bulma/sass/grid/columns.sass"
 {% endcapture %}
 
 {% capture columns %}
@@ -26,7 +26,7 @@ breadcrumb:
 {% endcapture %}
 
 {% capture only_button %}
-@import "bulma/sass/utilities/_all"
+@import "bulma/sass/utilities/_all.sass"
 @import "bulma/sass/elements/button.sass"
 {% endcapture %}