Description for version incorrect

[tedyoung]

In https://github.com/stCarolas/setup-maven/blob/master/action.yml#L6 it says

# Version Spec of the version to use. Examples: 10.x, 10.15.1, >=10.15.0

which is wrong and is confusing as it's what's pasted into the clipboard. Since the version number becomes the url, that description should be changed to reflect how it gets the maven version.