[css-counter-styles-3] A too-short 'symbols' descriptor just makes it not define a counter style, not invalid. Fixes #5698.

Author: tabatkins

css-counter-styles-3/Overview.bs

@@ -1048,7 +1048,7 @@ Marker characters: the 'symbols' and 'additive-symbols' descriptors</h3>
 	Some counter systems specify that the 'symbols' descriptor must have at least two entries.
 	If the counter style's system is such,
 	and the 'symbols' descriptor has only a single entry,
-	the descriptor is invalid and must be ignored.
+	the ''@counter-style'' rule does not define a [=counter style=].
 
 	Each entry in the 'symbols' descriptor's value defines a <dfn export>counter symbol</dfn>,
 	which is interpreted differently based on the counter style's system.