fix(checkbox): replaced inherit margin values with 0

Inherit is not the proper value, 0 should be used instead

Closes #6472

Author: EladBezalel

src/components/checkbox/checkbox.scss

@@ -23,12 +23,12 @@ md-checkbox {
   position: relative;
   min-width: $checkbox-width;
   min-height: $checkbox-width;
-  @include rtl(margin-left, inherit, $checkbox-margin);
-  @include rtl(margin-right, $checkbox-margin, inherit);
+  @include rtl(margin-left, 0, $checkbox-margin);
+  @include rtl(margin-right, $checkbox-margin, 0);
 
   &:last-of-type {
-    @include rtl(margin-left, inherit, 0);
-    @include rtl(margin-right, 0, inherit);
+    margin-left: 0;
+    margin-right: 0;
   }
 
   &.md-focused:not([disabled]) {