[css-cascade-3] fix typo noticed in AC review

Author: svgeesus

css-cascade-3/Overview.bs

@@ -987,7 +987,7 @@ Changes Since the 28 August 2018 Candidate Recommendation</h3>
 
 	<ul class=non-normative>
 		<li>
-			Clarify that ''@import'' ordering requirements are only in consideratin of valid rules.
+			Clarify that ''@import'' ordering requirements are only in consideration of valid rules.
 		<li>
 			Clarify that [=computed values=] are not serialization strings, but abstract data.
 	</ul>