app.py: all models multispeaker, fix instructions

app.py

@@ -25,7 +25,7 @@ def main():
 
             1. Write an utterance to generate,
             2. Select the model to synthesize with
-            3. Select speaker (only for multi-speaker models)
+            3. Select speaker
             4. Hit "Generate" and listen to the result!
 
             When you select model for the first time,