Use correct value for xs and sm breakpoints in documentation

[amygori]

The documentation on the available responsive utility classes includes an incorrect value for the xs max-width and sm min-width. This corrects the value and brings the available classes table values in line with the values elsewhere in the documentation (c.f. Responsive Breakpoints) and with the functionality of the classes themselves.

[mixalbl4-127]

@amygori
This fix (in 4.0.0-beta) breaks this mixins (in alpha-6 worked fine)

  @include media-breakpoint-only(xl)
  @include media-breakpoint-only(xs)

Temp solution:

  @include media-breakpoint-only(xl)   =>   @include media-breakpoint-up(xl)
  @include media-breakpoint-only(xs)   =>   @include media-breakpoint-down(xs)