update README to suggest correct java_subversion for local rpm file (Centos 7)

[francescor]

my settings for a local rpm file in files/jdk-8u181-linux-x64.rpm was not working (Centos 7)

# which version?
java_version: 8

# which subversion?
java_subversion: 181

jdk_tarball_file: jdk-8u181-linux-x64

java_download_path: /tmp
java_download_from_oracle: false
java_remove_download: false

java_set_javahome: true
java_install_jce: false

I've fixed it by replacing:

java_subversion: 181

with

java_subversion: 181-amd64