RFR: 7903067: JMH: Improve English grammar in error messages [v4]

[Aarre Laakso]

> I improved the English grammar in a few error messages

Aarre Laakso has updated the pull request incrementally with two additional commits since the last revision:

 - Change this error message to match grammar of other similar ones
 - Grammatical change to error message

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Changes:
  - all: https://git.openjdk.java.net/jmh/pull/56/files
  - new: https://git.openjdk.java.net/jmh/pull/56/files/99be8bfa..d1e6063d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmh&pr=56&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jmh&pr=56&range=02-03

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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