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

scss/mixins/_breakpoints.scss

@@ -2,7 +2,7 @@
 //
 // Breakpoints are defined as a map of (name: minimum width), order from small to large:
 //
-//    (xs: 0, sm: 544px, md: 768px)
+//    (xs: 0, sm: 576px, md: 768px)
 //
 // The map defined in the `$grid-breakpoints` global variable is used as the `$breakpoints` argument by default.
 
@@ -10,7 +10,7 @@
 //
 //    >> breakpoint-next(sm)
 //    md
-//    >> breakpoint-next(sm, (xs: 0, sm: 544px, md: 768px))
+//    >> breakpoint-next(sm, (xs: 0, sm: 576px, md: 768px))
 //    md
 //    >> breakpoint-next(sm, $breakpoint-names: (xs sm md))
 //    md
@@ -21,8 +21,8 @@
 
 // Minimum breakpoint width. Null for the smallest (first) breakpoint.
 //
-//    >> breakpoint-min(sm, (xs: 0, sm: 544px, md: 768px))
-//    544px
+//    >> breakpoint-min(sm, (xs: 0, sm: 576px, md: 768px))
+//    576px
 @function breakpoint-min($name, $breakpoints: $grid-breakpoints) {
   $min: map-get($breakpoints, $name);
   @return if($min != 0, $min, null);
@@ -31,7 +31,7 @@
 // Maximum breakpoint width. Null for the largest (last) breakpoint.
 // The maximum value is calculated as the minimum of the next one less 0.1.
 //
-//    >> breakpoint-max(sm, (xs: 0, sm: 544px, md: 768px))
+//    >> breakpoint-max(sm, (xs: 0, sm: 576px, md: 768px))
 //    767px
 @function breakpoint-max($name, $breakpoints: $grid-breakpoints) {
   $next: breakpoint-next($name, $breakpoints);
@@ -41,9 +41,9 @@
 // Returns a blank string if smallest breakpoint, otherwise returns the name with a dash infront.
 // Useful for making responsive utilities.
 //
-//    >> breakpoint-infix(xs, (xs: 0, sm: 544px, md: 768px))
+//    >> breakpoint-infix(xs, (xs: 0, sm: 576px, md: 768px))
 //    ""  (Returns a blank string)
-//    >> breakpoint-infix(sm, (xs: 0, sm: 544px, md: 768px))
+//    >> breakpoint-infix(sm, (xs: 0, sm: 576px, md: 768px))
 //    "-sm"
 @function breakpoint-infix($name, $breakpoints: $grid-breakpoints) {
   @return if(breakpoint-min($name, $breakpoints) == null, "", "-#{$name}");