Fix `Stdio::piped` example code and lint

Summary:
Invoking `rev` does not add a trailing newline when none is present in
the input (at least on my Debian). Nearby examples use `echo` rather
than `rev`, which probably explains the source of the discrepancy.

Also, a `mut` qualifier is unused.

Test Plan:
Copy the code block into <https://play.rust-lang.org> with a `fn main`
wrapper, and run it. Note that it compiles and runs cleanly; prior to
this commit, it would emit an `unused_mut` warning and then panic.

wchargin-branch: stdio-piped-docs

0 files changed, 0 insertions, 0 deletions