No `open_mode` method

[tesuji]

The chapter on File I/O mentions the open_mode method, but there aren't any
methods like that in the Rust Std documentation. Maybe it should be OpenOptions struct.

[mikong]

For reference, the removal of open_mode function in favor of OpenOptions struct is described in:

https://rust-lang.github.io/rfcs/0517-io-os-reform.html#files

We can see open_mode being marked as deprecated back in February 2015 for the reason "replaced with std::fs::OpenOptions":

rust-lang/rust@95d9046