Editorial: Fix ThrowCompletion link. (#1599)

Author: Ms2ger

docs/index.bs

@@ -53,6 +53,7 @@ spec: push; urlPrefix: https://w3c.github.io/push-api/
 spec: ecma-262; urlPrefix: http://tc39.github.io/ecma262/
     type: abstract-op
         text: NormalCompletion; url: sec-normalcompletion
+        text: ThrowCompletion; url: sec-throwcompletion
     type: dfn
         text: Assert; url: sec-algorithm-conventions
         text: [[Call]]; url: sec-ecmascript-function-objects-call-thisargument-argumentslist