[12] RFR: 8216176: Clarify the singleton description in j.t.c.JapaneseEra class

[Naoto Sato]

Hello,

Please review the change for the following issue:

Issue: https://bugs.openjdk.java.net/browse/JDK-8216176
CSR: https://bugs.openjdk.java.net/browse/JDK-8216177

The proposed changeset is located at:

http://cr.openjdk.java.net/~naoto/8216176/webrev.00/

This is a simple one line change that is clarifying the era singleton 
description, which was added with the fix to 8212941.

Naoto