Remove doubled braces in non_exhaustive structs’ documentation text.
Remove doubled braces in non_exhaustive structs’ documentation text.

In commit 4b806878549990d2ad2aa3c265751d3d89947cdf (part of Rust 1.52.1)
many calls to `write!(w,` were replaced with `w.write_str(`, but this
one contained braces that were doubled to escape them when taken as a
format string, and so changing the call without changing the text caused
them to become doubled in the final HTML output.

I examined `print_item.rs` and the diff of that prior commit for any
other occurrences of this mistake and I did not find any.

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust