fix docs links

Author: slhck

docs/index.md

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 ![Docker Image Version](https://img.shields.io/docker/v/slhck/ffmpeg-normalize?sort=semver&label=Docker%20image)
 ![GitHub Actions Workflow Status](https://img.shields.io/github/actions/workflow/status/slhck/ffmpeg-normalize/python-package.yml)
 
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 A utility for batch-normalizing audio using ffmpeg.
 
 This program normalizes media files to a certain loudness level using the EBU R128 loudness normalization procedure. It can also perform RMS-based normalization (where the mean is lifted or attenuated), or peak normalization to a certain target level.