RFR: CODETOOLS-7903567: JMH: Improve README guidance for archetype bootstrap

[abyrne133]

Attempting to execute the "mvn archetype:generate" command specified in Step 1 of Preferred Usage in the README will result in a maven execution error if you interpret it like I did! In my case I naively interpreted the following text to mean that I could create a sub project within the test folder of my already existing java project. "test" is the default test folder!

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> "Step 1. Setting up the benchmarking project. The following command will generate the new JMH-driven project in test folder:"

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This resulted in 10-15 minutes of trying to figure out if I needed to install something new to use mvn archetype - as I have not used it before.

This change might make it easier for users to get set up with JMH project templates.

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Commit messages:
 - Formatting of test word
 - Wording improved as per code review
 - README Preferred Usage Improvement

Changes: https://git.openjdk.org/jmh/pull/123/files
 Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=123&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903567
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jmh/pull/123.diff
  Fetch: git fetch https://git.openjdk.org/jmh.git pull/123/head:pull/123

PR: https://git.openjdk.org/jmh/pull/123