docs: specify how project is selected in cloud shell

This bug comes from go/coleleah-rust-friction-log

In the setting up Cloud Shell guide, before [this step](https://github.com/googleapis/google-cloud-rust/blob/7b40638a34e5c2ce0f3b359c563a1aa9e5bd76fe/guide/src/setting_up_rust_on_cloud_shell.md?plain=1#L78), specify how a user should select their Rust project in the UI - for those using the linked Cloud Shell Editor, they need to open the proper folder before they can edit their main code file