Fix the last line of README.md

[maxxyme]

Hello, the last line of the readme is a nonsense, it's a password but has a java command in it:
$ KEYSTORE_PASSWORD=foobar java -jar target/*.jar

https://github.com/spring-cloud-samples/configserver/blob/master/README.md

Cheers.

[spencergibb]

That's standard shell script to set an environment variable before a command

[maxxyme]

Not standard under Windows...

[spencergibb]

hence the $