fix(docs): correct spelling of 'ffmpeg' in installation instructions (#914)

Author: yongjincho

README.md

@@ -98,7 +98,7 @@ conda create -y -n lerobot python=3.10
 conda activate lerobot
 ```
 
-When using `miniconda`, if you don't have `fffmpeg` in your environment:
+When using `miniconda`, if you don't have `ffmpeg` in your environment:
 ```bash
 conda install ffmpeg
 ```