[web-animations] Fix a typo that I found while reading the spec.

No review, since it's trivial.

Author: emilio

web-animations-1/Overview.bs

@@ -3218,7 +3218,7 @@ a <a>target element</a> for which computed property values can be calculated.
         &lsquo;16px&rsquo;.
 
         If conflicts arise when expanding shorthand properties or replacing
-        logical properties with physical propertiess, apply the following rules
+        logical properties with physical properties, apply the following rules
         in order until the conflict is resolved:
 
         1.  Longhand properties override shorthand properties (e.g.