docs: Update outdated variable name in release process

[shorim]

Description

Changes proposed in this pull request (what was done and why):

• The variable defaultOtelImage was changed to defaultOTelCollectorImage in this PR. So, we need to update the release process documentation with the new variable name

Changes refer to particular issues, PRs or documents:

• feat: Remove unneeded operator flags #1527

Traceability

• The PR is linked to a GitHub issue.
• The follow-up issues (if any) are linked in the Related Issues section.
• If the change is user-facing, the documentation has been adjusted.
• If a CRD is changed, the corresponding Busola ConfigMap has been adjusted.
• The feature is unit-tested.
• The feature is e2e-tested.