libproc_macro docs: fix brace and bracket mixup

[steffengy]

The documentation indicates that brace is [.
Brace is mapped token::Brace which (expectedly) is {.
So the documentation is simply confusing brace and bracket there.

Even though it's just a very small issue, it can lead to quite some confusion.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

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[steffengy]

r? @alexcrichton (since @brson left)

[alexcrichton]

@bors: r+ rollup

Thanks!

[bors]

📌 Commit d46660e has been approved by alexcrichton