[doc] Update parallelism.mdx
- I found some mistakes and fixed them. sorry @stas00
- In addition, `DataParallel` and `DistributedDataParallel` are marked as code because they are class names. (I was trying to switch DataParallel to DataParallelism, but I realized that this means name of class)


## Reviewers
@stas00 
updated.<|||||>@stas00 This can be merged !