RFR: Misc fixes

[Jesper Wilhelmsson]

- Changes casing in headers to be consistent
- Fixed typos and minor cleanups
- Changes all JTReg to jtreg since that's how it's written in the official documentation
- Minor clarification in how to fix an incorrect backport

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Commit messages:
 - Misc fixes

Changes: https://git.openjdk.java.net/guide/pull/71/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=71&range=00
  Stats: 44 lines in 1 file changed: 3 ins; 0 del; 41 mod
  Patch: https://git.openjdk.java.net/guide/pull/71.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/71/head:pull/71

PR: https://git.openjdk.java.net/guide/pull/71