[fix] Index table topic label extraction

[sbreitbart-NOAA]

• Adjust how index table is labelled in create_rda() to resolve Table rda not produced when run inside withr::with_dir #372
• Make same adjustment to bnc table for consistency

[github-actions]

Code Metrics Report

Coverage	Code to Test Ratio	Test Execution Time
71.6%	1:0.2	8m48s

Code coverage of files in pull request scope (95.7%, patch 100.0%)

Files	Coverage	Patch Coverage
R/table_biomass_abundance_catch.R	95.6%	100.0%
R/table_index.R	95.8%	100.0%

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Reported by octocov