Update plugin id in README.
Update plugin id in README.

Due to the gradle plugin portal changes,
plugin id doesn't now have "gradle.plugin" prefix.

Signed-off-by: Yahor Berdnikau <65de286607a8090de37d6bb23a427517fd3c474e@gmail.com>

JLLeitschuh/ktlint-gradle,JLLeitschuh/ktlint-gradle