[web-animations-1] Fix a grammatical error

Author: birtles

web-animations-1/Overview.bs

@@ -3245,7 +3245,7 @@ is specified and the <em>next</em> keyframe in the list.
 The <a>timing function</a> specified on the last keyframe in the
 list is never applied.
 
-Each <a>keyframe</a> may has a <dfn>keyframe-specific composite
+Each <a>keyframe</a> may have a <dfn>keyframe-specific composite
 operation</dfn> that, if not null,  is applied to all values
 specified in that <a>keyframe</a>.
 The possible operations and their meanings are identical to those defined