fix case of slider theme

Author: andrewseguin

src/universal-app/theme.scss

@@ -1,5 +1,5 @@
 @import '../material/core/theming/all-theme';
-@import '../material-experimental/mdc-slider/mdc-slider';
+@import '../material-experimental/mdc-slider/slider-theme';
 @import '../material-experimental/mdc-theming/all-theme';
 @import '../material-experimental/mdc-typography/all-typography';