Correct regex to get a toolchain in rustic-doc/convert.sh

[dkislin]

I am not sure if it is me or something else I am doing wrong but it looks to me the regex is not entirely correct at least for rustup version I use:

rustic/rustic-doc/convert.sh

Line 5 in ad6f306

rustup show | sed -nr 's/(.*) \(default\)/\1/p' | head -n 1

# rustup --version
rustup 1.28.2 (2025-04-28)

# rustup show
Default host: aarch64-apple-darwin
rustup home:  /Users/ice/.rustup

installed toolchains
--------------------
stable-aarch64-apple-darwin (active, default)

active toolchain
----------------
name: stable-aarch64-apple-darwin
active because: it's the default toolchain
installed targets:
  aarch64-apple-darwin

So I'd say we need to add ".*" around default keyword in the regex.

Thank you in advance!