The only thing I could even guess without seeing your data before and after the formula, is to try this:
=LOWER(CONCATENATE("/",SUBSTITUTE(c2," ","- ") ,".html"))
or
=SUBSTITUTE(LOWER(CONCATENATE("/",C2,".html"))," ","- ")
If you could provide a sample piece before you run the formulas on it, and the expected results, then I could help further if this is not correct.