Update README.md

Use suggested composite command instead of 2 separate commands.

Author: Jake Shadle

README.md

@@ -66,7 +66,7 @@ Read ["Installing Rust"] from [The Book].
 
    ```sh
    # Update package mirrors (may be needed if you have a fresh install of MSYS2)
-   $ pacman -Sy&&pacman -S pacman-mirrors
+   $ pacman -Sy pacman-mirrors
    
    # Choose one based on platform:
    $ pacman -S mingw-w64-i686-toolchain