fix(button-group): fix nth-child index. (#13259)

Co-authored-by: Silvia Ivanova <[email protected]>
Co-authored-by: Simeon Simeonoff <[email protected]>

Author: 3 people

projects/igniteui-angular/src/lib/core/styles/components/button-group/_button-group-theme.scss

@@ -215,7 +215,7 @@
             }
         }
 
-        &:not(:nth-child(0)) {
+        &:not(:nth-child(1)) {
             margin-inline-start: rem(-1px);
         }
 
@@ -253,7 +253,7 @@
     }
 
     %igx-group-item-vertical {
-        &:not(:nth-child(0)) {
+        &:not(:nth-child(1)) {
             margin-top: rem(-1px);
             margin-inline-start: 0;
         }