RFR: Minor spelling correction in guide

[Mohamed Issa]

This changes fixes a typo in the **Reviewing and Sponsoring a Change** section of the guide.

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Commit messages:
 - Correct spelling - Reviever to Reviewer

Changes: https://git.openjdk.org/guide/pull/168/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=168&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/guide/pull/168.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/168/head:pull/168

PR: https://git.openjdk.org/guide/pull/168