Updates 'flow-tolerance' grammar to use <length-percentage [0,∞]> rather than <length-percentage>

Author: weinig

css-grid-3/Overview.bs

@@ -789,7 +789,7 @@ Placement Precision: the 'flow-tolerance' property</h3>
 
 	<pre class=propdef>
 	Name: flow-tolerance
-	Value: normal | <<length-percentage>> | infinite
+	Value: normal | <<length-percentage [0,∞]>> | infinite
 	Initial: normal
 	Percentages: relative to the [=grid-axis=] [=content box=] size of the [=grid lanes container=]
 	Inherited: no