Fix some incorrect promise stuff for streaming methods

They return ReadableStreams, never promises.```

Author: domenic

README.md

@@ -119,7 +119,7 @@ All three of the APIs support streaming output, via counterpart methods `summari
 ```js
 const writer = await Writer.create({ tone: "formal", length: "long" });
 
-const stream = await writer.writeStreaming(
+const stream = writer.writeStreaming(
   "A draft for an inquiry to my bank about how to enable wire transfers on my account"
 );
 

index.bs

@@ -2106,7 +2106,7 @@ Every [=interface=] [=interface/including=] the {{DestroyableModel}} interface m
 
   1. Let |compositeSignal| be the result of [=creating a dependent abort signal=] given |signals| using {{AbortSignal}} and |modelObject|'s [=relevant realm=].
 
-  1. If |compositeSignal| is [=AbortSignal/aborted=], then return [=a promise rejected with=] |compositeSignal|'s [=AbortSignal/abort reason=].
+  1. If |compositeSignal| is [=AbortSignal/aborted=], then throw |compositeSignal|'s [=AbortSignal/abort reason=].
 
   1. Let |stream| be a [=new=] {{ReadableStream}} created in |modelObject|'s [=relevant realm=].