typo in usage code in readme

[wyattscarpenter]

This is also pr #11

• node ./usavetool.js [mode: -sav-to-json, -json-to-sav] [input path] [output path?]

• node ./uesavetool.js [mode: -sav-to-json, -json-to-sav] [input path] [output path?]

[joric]

This is the first thing I ran into. Pls fix.