TableOfContents.md: mention rami3l/PLFaLean as a derived work

[rami3l]

Closes rami3l/PLFaLean#15.

As proposed in that very issue, this PR adds rami3l/PLFaLean as a derived work of PLFA.

@wadler Many thanks again for your appreciation!

[wenkokke]

Could you add a short sentence describing which version or commit your work is based on and which chapters you've translated?

[wadler]

@wenkokke Better to have incomplete information now than perfect information later, so I've hit "merge" for this.

@rami3l Thank you. Please do add the sentence suggested by Wen in a later pull request. I'm currently writing a book based on PLFA, but with Lean in place of Agda and appropriate changes to the text. I was going to use PLFL for my book, so could I ask you to choose a different name for your repository? Perhaps PLFA-in-Lean?

[rami3l]

I'm currently writing a book based on PLFA, but with Lean in place of Agda and appropriate changes to the text. I was going to use PLFL for my book, so could I ask you to choose a different name for your repository? Perhaps PLFA-in-Lean?

@wadler Thanks for letting me know! That said, I don't believe my personal exercise will really constitute a name conflict with your upcoming org, repo and/or book. If anything, I'd love to change the name of my repo to e.g. PLFaLean (so that it is clearly a PLFA derived work and has nothing to do with your new book), but I'd prefer to leave the project itself intact (the code is considered frozen so I'm a bit reluctant to change anything). How does that sound to you?

Could you add a short sentence describing which version or commit your work is based

@wenkokke Sorry, but I'm a bit confused right now because I'm not aware of any "version"s of this book. Has anything changed significantly since my finishing this project in 2023?

PS: If you are talking about GitHub releases, then it'll be https://github.com/plfa/plfa.github.io/releases/tag/v22.08, it seems to me.

[rami3l]

I have made the said changes and would like to re-request a review from both @wenkokke and @wadler. Many thanks to both of you in advance!