fix(category-chart): fix typography style references

Author: simeonoff

projects/igniteui-angular/src/lib/core/styles/components/charts/_category-chart-theme.scss

@@ -282,8 +282,8 @@
 
     $title-styles: igx-type-scale-category($type-scale, $title);
     $subtitle-styles: igx-type-scale-category($type-scale, $subtitle);
-    $x-axis-label-styles: igx-type-scale-category($type-scale, $x-axis-label);
-    $x-axis-title-styles: igx-type-scale-category($type-scale, $x-axis-title);
+    $x-axis-label-styles: igx-type-scale-category($type-scale, $x-axis-label-text-style);
+    $x-axis-title-styles: igx-type-scale-category($type-scale, $x-axis-title-text-style);
     $y-axis-label-styles: igx-type-scale-category($type-scale, $y-axis-label-text-style);
     $y-axis-title-styles: igx-type-scale-category($type-scale, $y-axis-title-text-style);