RFR: Improve English grammar in error messages

[Aarre Laakso]

I improved the English grammar in a few error messages

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Commit messages:
 - Improve English grammar in error messages

Changes: https://git.openjdk.java.net/jmh/pull/56/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmh&pr=56&range=00
  Stats: 6 lines in 3 files changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jmh/pull/56.diff
  Fetch: git fetch https://git.openjdk.java.net/jmh pull/56/head:pull/56

PR: https://git.openjdk.java.net/jmh/pull/56