Fix errors in README.md (coqui-ai#2478)

Running the sample code below results in an error `language_id = self.tts_model.language_manager.name_to_id[language_name]`.
The fix is running the code with the correct language strings, the readme has been updated in this PR to work.

I assume this small typo leads to coqui-ai#2456 and coqui-ai#2458

README.md

@@ -195,8 +195,8 @@ tts.tts_to_file(text="Ich bin eine Testnachricht.", file_path=OUTPUT_PATH)
 # Example voice cloning with YourTTS in English, French and Portuguese:
 tts = TTS(model_name="tts_models/multilingual/multi-dataset/your_tts", progress_bar=False, gpu=True)
 tts.tts_to_file("This is voice cloning.", speaker_wav="my/cloning/audio.wav", language="en", file_path="output.wav")
-tts.tts_to_file("C'est le clonage de la voix.", speaker_wav="my/cloning/audio.wav", language="fr", file_path="output.wav")
-tts.tts_to_file("Isso é clonagem de voz.", speaker_wav="my/cloning/audio.wav", language="pt", file_path="output.wav")
+tts.tts_to_file("C'est le clonage de la voix.", speaker_wav="my/cloning/audio.wav", language="fr-fr", file_path="output.wav")
+tts.tts_to_file("Isso é clonagem de voz.", speaker_wav="my/cloning/audio.wav", language="pt-br", file_path="output.wav")
 ```
 
 ### Command line `tts`