Replace java with javaw in README.md

From 24e4dd7 commit message: "I don't know how you built a native installer for Windows. So, I've used http://converticon.com/ to get a high-resolution jdotxt icon and created a shortcut to run "%JAVA_HOME%\bin\javaw -jar jdotxt.jar"... and it took me 15 minutes to understand that Windows kept on opening cmd.exe terminal window because I used java.exe instead of javaw.exe at first."

Also, I don't recall if javaw is present on non-Windows platforms. So, it would be great if you double-check this instruction on your machine before accepting it.