[css-transforms-2] Use <length-percentage>, simplify grammar a little bit.

Author: tabatkins

css-transforms-2/Overview.bs

@@ -37,7 +37,7 @@ Fluff here.
 
 <pre class="propdef">
 Name: translate
-Value: [ <<length>> | <<percentage>> ] [[ <<length>> | <<percentage>> ] <<length>>? ]?
+Value: <<length-percentage>>{1,2} <<length>>?
 Initial: 0
 Inherited: no
 Percentages: relative to the width of the containing block (for the first value) or the height (for the second value)