message: Fix readme image alt description
diff: <p align="center">
  <img width="600"
  - alt="Alacritty running vim inside tmux"
  + alt="Alacritty - A fast, cross-platform, OpenGL terminal emulator"
  src="https://user-images.githubusercontent.com/8886672/103264352-5ab0d500-49a2-11eb-8961-02f7da66c855.png">
  </p>
  