Clarify stacked-to-horizontal in layout/grid.md (twbs#25520)

- Fixes typo with "with at" in "horizontal with at the small breakpoint"

docs/4.0/layout/grid.md

@@ -272,7 +272,7 @@ For grids that are the same from the smallest of devices to the largest, use the
 
 ### Stacked to horizontal
 
-Using a single set of `.col-sm-*` classes, you can create a basic grid system that starts out stacked before becoming horizontal with at the small breakpoint (`sm`).
+Using a single set of `.col-sm-*` classes, you can create a basic grid system that starts out stacked and becomes horizontal at the small breakpoint (`sm`).
 
 <div class="bd-example-row">
 {% example html %}