[css-snap-size] Fix baseline formula

Author: kojiishi

css-snap-size/Overview.bs

@@ -63,8 +63,6 @@ the closest multiple of the unit.
 defines how to compute the line box height
 after the line box was constructed from inline-level boxes.
 The rounding is applied to the computed line box height.
-The additional spaces created by the rounding
-are added <a>over</a> and <a>under</a> the line box.
 
 The <a>grid baseline position</a> then determines
 how the additional spaces are distributed to <a>over</a> and <a>under</a>.
@@ -126,8 +124,8 @@ Snapping Baselines {#snap-baseline}
 When the <a>grid baseline position</a> is a positive number equals to or less than 1,
 the additional spaces are distributed using the following formula:
 
-* space-over = (BP - T &amp; GU + GU) &amp; GU
-* space-under = GU - (space-over + T + B) &amp; GU
+* space-over = BP - T % GU, add GU if negative
+* space-under = GU - (space-over + T + B) % GU
 * GU: the <a>grid unit</a>.
 * BP: the <a>grid baseline position</a> &times; GU / 100.
 * T: the distance between the top of the line box and the baseline.