[css-scroll-snap-1] Use the right term; 'reachable' is for a different concept that's not what we meant here.

Author: tabatkins

css-scroll-snap-1/Overview.bs

@@ -344,7 +344,7 @@ Scroll Snap Strictness: the ''scroll-snap-type/none'', ''proximity'', and ''mand
             If specified on a <a>scroll container</a>,
             the <a>scroll container</a> is required to be <a>snapped</a> to a snap position
             when there are no active scrolling operations.
-            If a reachable snap position exists
+            If a valid <a>snap position</a> exists
             then the scroll container must <a>snap</a> at the termination of a scroll
             (if none exist then no <a>snapping</a> occurs).