Update wav2vec README.md with decoding lexicon

[AndreasPlt]

Currently, there is no information about lexicons for wav2vec 2.0 decoding in the README. I added both a download link for KenLM lexicon (taken from #2502) and the corresponding argument in the command.

Before submitting

• [x] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• [x] Did you read the contributor guideline?
• [x] Did you make sure to update the docs?
• [x] Did you write any new necessary tests?

What does this PR do?

Implements README update discussed in #2502.

PR review

Anyone in the community is free to review the PR once the tests have passed. If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Absolutely!