std::io: Use re-exported pathes in examples.

We use re-exported pathes (e.g. std::io::Command) and original ones
(e.g. std::io::process::Command) together in examples now. Using
re-exported ones consistently avoids confusion.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #15206 : omasanori/rust/use-reexported, r=alexcrichton

We use re-exported pathes (e.g. std::io::Command) and original ones
(e.g. std::io::process::Command) together in examples now. Using
re-exported ones consistently avoids confusion.