Update bootstrap/scss/mixins/_breakpoints.scss (#21285)
Update bootstrap/scss/mixins/_breakpoints.scss (#21285)

grid-breakpoint for sm is 576px
https://github.com/twbs/bootstrap/blob/v4-dev/scss/_variables.scss#L186-L192

1._breakpoints.scss
comment  says that grid-breakpoint for sm is 544px,

2.http://v4-alpha.getbootstrap.com/layout/overview/#responsive-breakpoints
current document says that grid-breakpoint for sm is 544px,
but it should be 576px
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