Update README.md to fix lang_code error

[zboyles]

The example fails if using the Kokoro model it is written with. I corrected it to resolve the issue and made a few minor changes. Feel free to revert those changes if you want.

Changes

• Set lang_code to a and added a short explaination
• Added 2nd line to text to demonstrate the join_audio feature
• Changed audio_format from its default wav to mp3

[Blaizzy]

Thanks for the fix, indeed language code should be:

# 🇺🇸 'a' => American English, 🇬🇧 'b' => British English
# 🇯🇵 'j' => Japanese: pip install misaki[ja]
# 🇨🇳 'z' => Mandarin Chinese: pip install misaki[zh]
# 🇪🇸 'e' => Spanish es
# 🇫🇷 'f' => French fr-fr
# 🇮🇳 'h' => Hindi hi
# 🇮🇹 'i' => Italian it
# 🇧🇷 'p' => Brazilian Portuguese pt-br