[css-multicol] make the 1em value of normal mandatory as per WG resolution and existing interop

Author: rachelandrew

css-multicol-1/Overview.bs

@@ -619,8 +619,7 @@ Column gaps and rules</h2>
 		<dt><dfn>normal</dfn>
 		<dd>
 			Identical to <a value for="column-gap">&lt;length-percentage></a>,
-			but with a UA-specified length.
-			A value of ''1em'' is suggested.
+			but with a used value of ''1em''.
 	</dl>