[css-overflow-3] Fix mistaken initial value

overvflow-block and overflow-inline are longhands of overflow, whose
initial value is visible. So listing them as having an initial value
of auto made no sense.

css-overflow-3/Overview.bs

@@ -491,7 +491,7 @@ as the Flow-Relative box model properties defined in [[css-logical-1#box]].
   <pre class="propdef">
   Name: overflow-inline, overflow-block
   Value: <<'overflow'>>
-  Initial: auto
+  Initial: visible
   Applies to: Same as 'overflow-x' and 'overflow-y'
   Inherited: No
   Percentages: N/A