[css-grid-1] Typo. Fixes #4994

Author: fantasai

css-grid-1/Overview.bs

@@ -1414,7 +1414,7 @@ The Explicit Grid</h3>
 	while negative indexes count from the <a>end</a> side
 	(starting from -1 for the <a>end</a>-most <a>explicit</a> line).
 
-	The 'grid' and 'grid-template' properties are a <a>shorthands</a>
+	The 'grid' and 'grid-template' properties are <a>shorthands</a>
 	that can be used to set all three <dfn>explicit grid properties</dfn>
 	('grid-template-rows', 'grid-template-columns', and 'grid-template-areas')
 	at the same time.