fix(button-toggle): not setting proper border in vertical mode (#13397)

Looks like something that slipped through when refactoring the previous PR to support multiple appearances. The selector that was supposed to set the proper borders in vertical mode was incorrect.

Author: crisbeto

src/lib/button-toggle/_button-toggle-theme.scss

@@ -44,10 +44,12 @@
     border-right: solid 1px $divider-color;
   }
 
-  .mat-button-toggle-vertical-appearance-standard .mat-button-toggle + .mat-button-toggle {
-    border-left: none;
-    border-right: none;
-    border-top: solid 1px $divider-color;
+  .mat-button-toggle-group-appearance-standard.mat-button-toggle-vertical {
+    .mat-button-toggle + .mat-button-toggle {
+      border-left: none;
+      border-right: none;
+      border-top: solid 1px $divider-color;
+    }
   }
 
   .mat-button-toggle-checked {