Fix badly named variable

[pinin4fjords]

Fixing an error reported in nf-core/rnaseq#1390, caused by a mistake in variable naming.

PR checklist

Closes #XXX

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
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    • nf-core subworkflows test <SUBWORKFLOW> --profile conda

[pinin4fjords]

@maxulysse @SPPearce For some reason the singularity test was hanging here, on SALMON_QUANT. I cancelled it and it went through. The same happened in the merge queue.

I have no reason to think that anything in this change would have triggered that (it's trivial), but maybe a problem has developed in the CI, which we should look out for.

[SPPearce]

Yes, we seem to have a general issue with pulling singularity containers at the moment