Discrimination for saying that people should comb their hair? You just assumed the boss thinks only straight
hair is professional, that is NOT what the OP said and you cannot found a case on an assumption. Sounds more like to me he thinks only tidy or groomed
hair is acceptable.
Many people think curly hair can or should be brushed, plenty of trained hairdressers are ignorant enough to comb us out into a huge pouf, and this is a man who likely knows even less about hair than a woman does! To me the comment was more stupidity than anything, combing our hair gives us anything but a "neat appearance
Nobody has pretended it was acceptable to say, we have all said it is unacceptable given the context and the manner in which it was done. It is acceptable to for a manager to remind the staff about personal grooming IF it is done in a professional manner, this was not. It would have been closer to discrimination if the manager had pulled the OP aside and only spoken to them, yet other people also had untidy hair and were not reminded.