[css-transforms-1] Change one occurrence of <percentage> to <length-percentage> per issue 646.

Author: smfr

css-transforms-1/Overview.bs

@@ -306,7 +306,7 @@ If only one value is specified, the second value is assumed to be <a value for=t
 If two or more values are defined and either no value is a keyword, or the only used keyword is <a value for=transform-origin>center</a>, then the first value represents the horizontal position (or offset) and the second represents the vertical position (or offset). A third value always represents the Z position (or offset) and must be of type <<length>>.
 
 <dl dfn-for="transform-origin" dfn-type="value">
-	: <<percentage>>
+	: <<length-percentage>>
 	:: A percentage for the horizontal offset is relative to the width of the [=reference box=]. A percentage for the vertical offset is relative to the height of the [=reference box=]. The value for the horizontal and vertical offset represent an offset from the top left corner of the [=reference box=].
 	: <<length>>
 	:: A length value gives a fixed length as the offset. The value for the horizontal and vertical offset represent an offset from the top left corner of the [=reference box=].