Fix typo in md document

[Shengliang]

• Fix typo in md document, missing "-".
• Add +nightly example since examples need nightly build version.

[rust-highfive]

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

Please see the contribution instructions for more information.

[adamgreig]

Thanks for the PR! The single-dash version of the arguments used to be correct but changed in a recent binutils release, so it's led to a lot of fixes like this being required!

For the rustup default, are you sure that's the right command? I thought either you want to ask rustup to install nightly (rustup install nightly) so that cargo +nightly build works, or maybe to use nightly by default (rustup default nightly) after installing it, but not to use nightly rustup to set the default to stable, which is what rustup +nightly default stable does:

$ rustup +nightly default stable
info: using existing install for 'stable-x86_64-unknown-linux-gnu'
info: default toolchain set to 'stable-x86_64-unknown-linux-gnu'

  stable-x86_64-unknown-linux-gnu unchanged - rustc 1.59.0 (9d1b2106e 2022-02-23)

info: note that the toolchain 'nightly-x86_64-unknown-linux-gnu' is currently in use (overridden by +toolchain on the command line)

$ rustc --version
rustc 1.59.0 (9d1b2106e 2022-02-23)

[Shengliang]

rustup use "+nightly" to select nightly version dynamically.

The default rustc version does not work (that I faced), I have to use "nightly" version.
Hence, I added example to install nightly dependencies.

$ rustc -V
$ rustc +nightly -V

$ rustup target add thumbv7m-none-eabi
$ rustup +nightly target add thumbv7m-none-eabi

[Shengliang]

Updated readme.

$ rustup default stable
info: using existing install for 'stable-x86_64-unknown-linux-gnu'
info: default toolchain set to 'stable-x86_64-unknown-linux-gnu'

stable-x86_64-unknown-linux-gnu unchanged - rustc 1.59.0 (9d1b2106e 2022-02-23)

$ rustc -V
rustc 1.59.0 (9d1b2106e 2022-02-23)
$ rustc +nightly -V
rustc 1.61.0-nightly (9c06e1ba4 2022-03-29)

$ rustup default nightly
info: using existing install for 'nightly-x86_64-unknown-linux-gnu'
info: default toolchain set to 'nightly-x86_64-unknown-linux-gnu'

nightly-x86_64-unknown-linux-gnu unchanged - rustc 1.61.0-nightly (9c06e1ba4 2022-03-29)

$ rustc -V
rustc 1.61.0-nightly (9c06e1ba4 2022-03-29)