[css-ruby] Add missing punctuation

Author: frivoal

css-ruby-1/Overview.bs

@@ -667,7 +667,7 @@ Ruby Layout</h2>
 	When a ruby structure is laid out,
 	its [=base level=] is laid out on the line
 	exactly as if its [=ruby bases=] were a regular sequence of [=inline boxes=]
-	and the [=ruby container=] a regular [=inline box=] wrapped around it
+	and the [=ruby container=] a regular [=inline box=] wrapped around it.
 
 	Each [=base container=] is then sized and positioned
 	to contain exactly all of its [=ruby bases=]’ margin boxes