fix(readme): changes hyperlink to the CLI tool

[xpanvictor]

The cli tool is in another repository https://github.com/OffchainLabs/cargo-stylus so pointed the link there.

Description

The previous relative url returned a 404 on github.
Changed

see the [Cargo Stylus CLI Tool](CargoStylus)

to

see the [Cargo Stylus CLI Tool](https://github.com/OffchainLabs/cargo-stylus)

which is the url to the cargo stylus cli

Checklist

• I have documented these changes where necessary.
• I have read the DCO and ensured that these changes comply.
• I assign this work under its open source licensing.

[rachel-bousfield]

LGTM, thanks!