HDDS-12647. Update testing instructions

[octachoron]

What changes were proposed in this pull request?

While testing HDDS-12499, some deviations from the testing instructions in CONTRIBUTING.md were necessary to get a successful Ozone CI run. This PR proposes to make them official:

• Fix a trivial typo in the image reference.
• Update the instruction on changing the image reference, in line with HDDS-11813 moving it to check.yml.
• Update the commit hash the pom.xml link points to. (Rationale: check.yml does not exist in the original commit, so that link has to be updated. This point restores consistency.)

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-12647

How was this patch tested?

• CI: https://github.com/octachoron/ozone-docker-runner/actions/runs/13955519342
• Ozone CI run after following the testing instructions with the proposed changes: https://github.com/octachoron/ozone/actions/runs/13955751134

[adoroszlai]

Thanks @octachoron for the fix.