[css-transforms-2] Add missing [] in syntax definition of perspective().

Fixes w3c#7582.

Author: dbaron

css-transforms-2/Overview.bs

@@ -1000,7 +1000,7 @@ In the following <dfn export>3d transform functions</dfn>, a <<zero>> behaves th
 :: same as ''rotate3d(0, 1, 0, &lt;angle>)''.
 : <span class='prod'><dfn>rotateZ()</dfn> = rotateZ( [ <<angle>> | <<zero>> ] )</span>
 :: same as ''rotate3d(0, 0, 1, &lt;angle>)'', which is a 3d transform equivalent to the 2d transform ''rotate(&lt;angle>)''.
-: <span class='prod'><dfn>perspective()</dfn> = perspective( <<length [0,∞]>> | <dfn id="valdef-perspective-func-none" dfn-type="value" for="perspective()">none</dfn> )</span>
+: <span class='prod'><dfn>perspective()</dfn> = perspective( [ <<length [0,∞]>> | <dfn id="valdef-perspective-func-none" dfn-type="value" for="perspective()">none</dfn> ] )</span>
 :: specifies a <a href="#PerspectiveDefined">perspective projection matrix</a>. This matrix scales points in X and Y based on their Z value, scaling points with positive Z values away from the origin, and those with negative Z values towards the origin. Points on the z=0 plane are unchanged. The parameter represents the distance of the z=0 plane from the viewer. Lower values give a more flattened pyramid and therefore a more pronounced perspective effect. For example, a value of 1000px gives a moderate amount of foreshortening and a value of 200px gives an extreme amount.
 
 	If the depth value is less than ''1px'',