polyg param improvement

[jfy133]

This PR is a minor clarification fix, to make clearer what the current complexity filter does.

It changes --complexity_filter to --complexity_filter_poly_g, as fastP offers proper whole-read complexity filtering (which we may want to consider later).

I've already manually tested this on my own machine and it appeared to work. All I did was appended the params/channels accordingly.

(first code PR - hope it's not a dumpster fire ;))

PR checklist

• 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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• Ensure the test suite passes (nextflow run . -profile test,docker).
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• Documentation in docs is updated
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• README.md is updated

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