Incorrect Docs for Customizing Grid Breakpoints

Created by: jpulec

The docs for customizing grid breakpoints show 3 tiers, with the following.

$grid-breakpoints: (
  sm: 480px,
  md: 768px,
  lg: 1024px
);

However this will result in a warning since bootstrap checks that the first $grid-breakpoint starts with zero after the introduction of #19308. The docs should probably be corrected to indicate this.