Fix documentation about how to run parallel test-build-profiles jobs

Santiago R.R.requested to merge
santiago/pipeline:fix-documentation-profile-matrix into master

The array suggested in the documentation was wrong, since it produced a single test-build-profiles job, taking into account the first variable definition.

Example: https://salsa.debian.org/gioele/libtool/-/pipelines/986665

/cc @gioele

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