Update archetype version in README.md as 0.9.9 will not be found

Maven gives the following error when trying to use archetype 0.9.9 but only have 1.0.0 installed.

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-archetype-plugin:2.2:generate (default-cli) on project standalone-pom: The desired archetype does not exist (com.badlogic.gdx:gdx-archetype:0.9.9) -> [Help 1]