[css-align] Fix the grammar for justify/align-content to use \!, so it's less confusing.

Author: tabatkins

css-align/Overview.bs

@@ -456,7 +456,7 @@ Content Distribution: the 'justify-content' and 'align-content' properties</h2>
 
 	<pre class="propdef">
 	Name: justify-content, align-content
-	Value: auto | <<baseline-position>> | [ <<content-distribution>> <<content-position>>? | <<content-position>> ] && <<overflow-position>>?
+	Value: auto | <<baseline-position>> | [ <<content-distribution>>? && <<content-position>>? ]! && <<overflow-position>>?
 	Initial: auto
 	Applies to: block containers, flex containers, and grid containers
 	Inherited: no