warning when custom workflow name contains special characters

[mirpedrol]

Close #2366

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 #2401 (9569144) into dev (13d17b8) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #2401      +/-   ##
==========================================
- Coverage   71.22%   71.15%   -0.08%     
==========================================
  Files          87       87              
  Lines        9410     9411       +1     
==========================================
- Hits         6702     6696       -6     
- Misses       2708     2715       +7     

Files Changed	Coverage Δ
nf_core/create.py	83.21% <100.00%> (-2.45%)	⬇️