Fix typo in README

[jcottom]

The README mentions a PLAYDATE_SDK environment variable where it should read PLAYDATE_SDK_PATH. This tripped me up when I tried to set up crank and crankstart and I found the original error message cryptic:

$ crank run --release --example hello_world
    Finished `release` profile [optimized] target(s) in 0.04s
Error: No such file or directory (os error 2)

With this change, it now shows:

$ crank run --release --example hello_world
    Finished `release` profile [optimized] target(s) in 0.01s
Error: Command failed: "/home/jack/Developer/PlaydateSDK/bin/pdc" "--strip" "/home/jack/repos/crankstart/target/Hello World" "/home/jack/repos/crankstart/target/Hello World.pdx"

Caused by:
    No such file or directory (os error 2)

which is a little more legible.

[boozook]

Thanks for the fix. But seems to there is missed import.
That's strange because it's on windows only 🤷🏻‍♂️.