Typo in boards/rp-pico/README.md

[miguelalizo]

### From Scratch

To start a basic project from scratch, create a project using `cargo new project-name`. Within the
project directory, run `cargo add rp-pico`, `cargo add cortex-m-rt`, and `cargo add panic-halt`. The
first command will <**add**> this HAL (Hardware Abstraction Layer), the second is required for the `#[entry]` macro, and _panic-halt_ creates a simple panic function, which just halts.

The word add was missing

[miguelalizo]

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