message: Fix typo in `README`
diff: [![Crates.io](https://img.shields.io/crates/v/iced.svg)](https://crates.io/crates/iced)
  [![License](https://img.shields.io/crates/l/iced.svg)](https://github.com/hecrj/iced/blob/master/LICENSE)
  -An GUI runtime for Rust, heavily inspired by Elm.
  +A GUI runtime for Rust, heavily inspired by Elm.
  [![GUI][gui_gif]][gui_gfycat]
  