doc: fix examples in README

[seriar]

Problem

The one-time snippet does not work as is and complains that the speaker is not defined

 > initialization of speaker-embedding layers.
 > Text: Перевірка мікрофона
 > Text splitted to sentences.
['Перевірка мікрофона']
Traceback (most recent call last):
  File "/home/serg/.local/bin/tts", line 8, in <module>
    sys.exit(main())
  File "/home/serg/.local/lib/python3.8/site-packages/TTS/bin/synthesize.py", line 350, in main
    wav = synthesizer.tts(
  File "/home/serg/.local/lib/python3.8/site-packages/TTS/utils/synthesizer.py", line 228, in tts
    raise ValueError(
ValueError:  [!] Look like you use a multi-speaker model. You need to define either a `speaker_name` or a `speaker_wav` to use a multi-speaker model.

Also it speakers.pth should be downloaded.

Fix

Just a few documentation changes:

• make instructions on what to download from Releases more precise
• add --speaker_id argument with one of the speakers

[seriar]

@robinhad Hi, here is a suggestion on how to improve the documentation. The original snippet for a one-time run did not work for me and I had to do some changes to make it work. It could be helpful for others

[robinhad]

Thank you @seriar! I also believe this whole section needs to be updated to highlight that you will use model without normalizer and autostress feature.