[css-position-3] Fix markup error.

Author: fantasai

css-position-3/Overview.bs

@@ -1707,7 +1707,7 @@ Acknowledgments</h2>
 			to, by default, constrain the box to its [=containing block=].
 			See [[#abspos-layout]].
 			(<a href="https://github.com/w3c/csswg-drafts/issues/10316">Issue 10316</a>)
-		<li>Specified the [=static-position rectangle=] of [=top layer boxes=] to be the [=initial containing block=].
+		<li>Specified the [=static-position rectangle=] of [=top layer=] boxes to be the [=initial containing block=].
 			(<a href="https://github.com/w3c/csswg-drafts/issues/9939">Issue 9939</a>)
 		<li>Clarified that the [=fixed positioning containing block=]
 			corresponds to the [=layout viewport=] (specifically) in [=continuous media=].