Update regex in readme test of nf-core lint

[ErikDanielsson]

Updated the Nextflow installation link in the readme test of nf-core lint to agree to the link in the pipeline template. Should close #1260.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1261 (30efe6e) into dev (4c8886e) will increase coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head 30efe6e differs from pull request most recent head 5989fe9. Consider uploading reports for the commit 5989fe9 to get more accurate results

@@            Coverage Diff             @@
##              dev    #1261      +/-   ##
==========================================
+ Coverage   67.39%   67.45%   +0.05%     
==========================================
  Files          50       50              
  Lines        5607     5607              
==========================================
+ Hits         3779     3782       +3     
+ Misses       1828     1825       -3     

Impacted Files	Coverage Δ
nf_core/lint/readme.py	79.31% <100.00%> (+10.34%)	⬆️
nf_core/lint_utils.py	87.09% <0.00%> (ø)

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