[css-grid] Fix tpoys and off-by-1s.

Author: tabatkins

css-grid/Overview.bs

@@ -2069,7 +2069,7 @@ Grid Definition Shorthand: the 'grid' property</h2>
 Clamping Overlarge Grids</h2>
 
 	Since memory is not infinite,
-	UAs may clamp the the possible size of the <a>grid</a>
+	UAs may clamp the possible size of the <a>grid</a>
 	to within a UA-defined limit,
 	dropping all lines outside that limit.
 	If a grid item is placed outside this limit,
@@ -2079,7 +2079,7 @@ Clamping Overlarge Grids</h2>
 	* If the <a>grid area</a> would span outside the limited grid,
 		its span is clamped to the last line of the limited <a>grid</a>.
 	* If the <a>grid area</a> would be placed completely outside the limited grid
-		is span must be truncated to 1
+		its span must be truncated to 1
 		and the area repositioned into the last <a>grid track</a> on that side of the grid.
 
 	<div class='example'>
@@ -2097,8 +2097,8 @@ Clamping Overlarge Grids</h2>
 
 		<pre class="lang-css">
 		.grid-item {
-			grid-row: 500 / 1000;
-			grid-column: 999 / 1000;
+			grid-row: 500 / 1001;
+			grid-column: 1000 / 1001;
 		}
 		</pre>
 	</div>