Sequences filename differs from documentation in README.md

[alurqu]

it's a minor issue, but README.md refers to the consensus sequence file pango_consensus_sequences.fasta.zstd as pango_consensus_sequences.fasta.zst in 'zstdcat data/pango_consensus_sequences.fasta.zst > pango_lineages.fasta'. The extensions '.zstd' and '.zst' don't match.