RFR: Cleanup contractions

[Jesper Wilhelmsson]

An attempt to unify the language in the guide with regards to contractions. There was a (very brief) discussion on this on the mailing list: https://mail.openjdk.java.net/pipermail/guide-dev/2020-December/000444.html

is not --> isn't
do not --> don't
are not --> aren't
etc

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Commit messages:
 - Merge
 - Change is not to isn't etc

Changes: https://git.openjdk.java.net/guide/pull/42/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=42&range=00
  Stats: 31 lines in 1 file changed: 0 ins; 0 del; 31 mod
  Patch: https://git.openjdk.java.net/guide/pull/42.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/42/head:pull/42

PR: https://git.openjdk.java.net/guide/pull/42