Fix typo in CounterDefinition.cs

Change "coutner" to "counter"

Author: ericmutta

src/AngleSharp.Css/Values/Primitives/CounterDefinition.cs

@@ -73,7 +73,7 @@ public Boolean Equals(CounterDefinition other) =>
 
         /// <summary>
         /// Checks for equality against the given object, if
-        /// the provided object is no coutner definition the
+        /// the provided object is no counter definition the
         /// result is false.
         /// </summary>
         /// <param name="obj">The object to check against.</param>