Fix 'Potential spelling errors' warning

[richelbilderbeek]

Fix 'Potential spelling errors' warning, which can be found e.g. here:

* checking tests ...
  Running ‘spelling.R’
  Comparing ‘spelling.Rout’ to ‘spelling.Rout.save’ ...7,14d6
< Potential spelling errors:
<   WORD          FOUND IN
< CMD           README.md:9,10
< MHCpan        read_netMHCpan_xls_output.Rd:5,13
< NetMHCIIpan   README.md:12,33,36,109,123,167,170
< cgi           demo.Rmd:32
< nph           demo.Rmd:32
< sw            demo.Rmd:32

So, either one can fix these spelling errors, or add the words to the whitelist/allowslist in inst/WORDLIST :-)