Editorial: mark generatorBody() as a completion record in GeneratorStart (#3597)

Author: michaelficarra

spec.html

@@ -49808,7 +49808,7 @@ <h1>
               1. Let _result_ be Completion(Evaluation of _generatorBody_).
             1. Else,
               1. Assert: _generatorBody_ is an Abstract Closure with no parameters.
-              1. Let _result_ be _generatorBody_().
+              1. Let _result_ be Completion(_generatorBody_()).
             1. Assert: If we return here, the generator either threw an exception or performed either an implicit or explicit return.
             1. Remove _acGenContext_ from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
             1. Set _acGenerator_.[[GeneratorState]] to ~completed~.