[css-grid] fix typo: s/streched/stretched/ (#748)

* [css-transforms] Replaced <length> | <percentage> by <length-percentage>

* Fixed typo in hint to 'auto' value (fixes #747)

Author: SebastianZ

css-grid/Overview.bs

@@ -1398,7 +1398,7 @@ Explicit Track Sizing: the 'grid-template-rows' and 'grid-template-columns' prop
 			of the <a>grid items</a> occupying the <a>grid track</a>.
 
 			Note: ''grid-template-rows/auto'' track sizes (and only ''grid-template-rows/auto'' track sizes)
-			can be streched by the 'align-content' and 'justify-content' properties.
+			can be stretched by the 'align-content' and 'justify-content' properties.
 
 		<dt><dfn lt="fit-content()">fit-content( [ <<length>> | <<percentage>> ] )</dfn>
 		<dd>