message: Fix target installation instruction
diff: @@ -18,7 +18,7 @@ This project is nascent and still under heavy development, but first steps:
  1. Add dependencies for cross-compilation.
  ```bash
  - rustup target add thumbv7m-none-eabi
  + rustup target add thumbv7em-none-eabi
  ```
  1. Use the `run_example` script to compile and run the example app you want
  