Fix the typo in --with-liburcu=mb pairwise definition

It was "ucru," but it should be "urcu". The pairwise CI job fails as a result with:

configure: error: unrecognized options: --with-libucru

Validation job: https://gitlab.isc.org/isc-projects/bind9/-/jobs/3352264

Edited by Michal Nowak

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