docs/ci/predefined_variables: fix variable name in example

What does this MR do?

This is a trivial typo, the documentation refers to CI_PROJECT_NAME:

For example, if CI_PROJECT_NAME is root-group/child-group/grandchild-group, CI_PROJECT_ROOT_NAMESPACE would be root-group.

instead of CI_PROJECT_NAMESPACE:

For example, if CI_PROJECT_NAMESPACE is root-group/child-group/grandchild-group, CI_PROJECT_ROOT_NAMESPACE would be root-group.

Related issues

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/cc @marcel.amirault

Labels for this MR: documentation devopsverify ~"group::continuous integration" docsfix