Update _custom-forms.scss (#31693)

Typo in box-shadow attribute, global variable used instead of local.

Co-authored-by: Arthur Shlain <artur.slain@gmail.com>
Co-authored-by: XhmikosR <xhmikosr@gmail.com>

scss/_custom-forms.scss

@@ -38,7 +38,7 @@
   &:focus ~ .custom-control-label::before {
     // the mixin is not used here to make sure there is feedback
     @if $enable-shadows {
-      box-shadow: $input-box-shadow, $input-focus-box-shadow;
+      box-shadow: $input-box-shadow, $custom-control-indicator-focus-box-shadow;
     } @else {
       box-shadow: $custom-control-indicator-focus-box-shadow;
     }