ARROW-6050: [Java] Update out-of-date java/flight/README.md
ARROW-6050: [Java] Update out-of-date java/flight/README.md

Related to [ARROW-6050](https://issues.apache.org/jira/browse/ARROW-6050).

Closes #4959 from tianchen92/ARROW-6050 and squashes the following commits:

6f29545d1 <tianchen> ARROW-6050:  Update out-of-date java/flight/README.md

Authored-by: tianchen <f328ad194fc0e3fc13d1a6206a6c280c752b1581@alibaba-inc.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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