[css-forms-1] Change gap to 0.5em (#12139)

This matches the implementation and HTML spec PR: whatwg/html#10670

Author: lukewarlow

css-forms-1/Overview.bs

@@ -904,7 +904,7 @@ input:is([type="color"], [type="button"], [type="reset"], [type="submit"]) {
   /* Push picker icon to the right of the box and have some space
    * in between it and the text. */
   display: inline-flex;
-  gap: 1em;
+  gap: 0.5em;
 
   user-select: none;