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ARROW-5014: [Java] Fix typos in Flight module

This fixes a few typos that I found while going through the Java Flight module.

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #4037 from BryanCutler/java-fix-flight-typos-ARROW-5014 and squashes the following commits:

4eb9912 <Bryan Cutler> fix typos in java flight module