Update examples to use the same Rust version used for the parser

[Tartasprint]

In pest-parser/pest#715 the Rust edition was updated to the 2021 edition.
I imagine it could be a good thing to the same with the examples given in the book.

This is the first time I contribute to a project, I am not sure how to proceed. I forked this repository and made the changes to adress this issue on this branch. Now I think I should do a pull request, but since I'm not sure if those changes are wanted I prefered creating an issue. Please tell me if this the expected behaviour :D

[tomtau]

@Tartasprint yes, feel free to go ahead to open a PR. Maybe pest versions could be bumped in those examples as well?

[Tartasprint]

I can do that :)
Shouldn't I also update the rust-version field to be 1.59.0 according to this pull request ?

[tomtau]

@Tartasprint no, as I mentioned, that was for cargo-msrv (https://foresterre.github.io/cargo-msrv/), not pest itself