message: Fix `README` or `arc` example
diff: -## arc_to
  +## Arc
  An application that uses the `Canvas` widget to draw a rotating arc.
  @@ -8,7 +8,7 @@ The __[`main`]__ file contains all the code of the example.
  You can run it with `cargo run`:
  ```
  -cargo run --package arc_to
  +cargo run --package arc
  ```
  [`main`]: src/main.rs
  