[css-grid] Fix grammatical error

Author: fantasai

css-grid/Overview.bs

@@ -660,7 +660,7 @@ Sizing Grid Containers</h3>
 <h2 id="grid-items">
 Grid Items</h2>
 
-	The contents of a <a>grid container</a> consists of zero or more <dfn export id="grid-item" lt="grid item">grid items</dfn>:
+	The content of a <a>grid container</a> consists of zero or more <dfn export id="grid-item" lt="grid item">grid items</dfn>:
 	each child of a <a>grid container</a>
 	becomes a <a>grid item</a>,
 	and each contiguous run of text that is directly contained inside a <a>grid container</a>