my-env/default.nix: fix "sdl-env" vs "env-sdl" typo

my-env/default.nix lists an example on how to create an SDL environment
that can be installed with "nix-env -i sdl-env". That is actually wrong
and will not work. The correct command is "nix-env -i env-sdl".
Merge pull request #333 from bjornfor/fix-typo

my-env/default.nix: fix "sdl-env" vs "env-sdl" typo