[css-grid-1] fix typo

Author: vwkd

css-grid-1/Overview.bs

@@ -3264,7 +3264,7 @@ Grid Item Placement Algorithm</h3>
 			</pre>
 
 			The number of columns needed is 6.
-			The <a>explicit grid</a> provides its 5 columns
+			The <a>explicit grid</a> provides 5 columns
 			(from 'grid-template-columns')
 			with lines number 1 through 6,
 			but <code>#grid-item</code>’s column position means it ends on line 7,