[css-grid] Fix typos. (Fixes #835)

Author: fantasai

css-grid/Overview.bs

@@ -2458,7 +2458,7 @@ Subgrids</h2>
 				</pre>
 
 				If the subgrid has margins/borders/padding,
-				the the size of those margins/borders/padding also influence sizing.
+				the size of those margins/borders/padding also influences sizing.
 				For example, if the subgrid has 100px padding:
 
 				<pre>#subgrid { padding: 100px; }</pre>