Hi!
As far as I know, this really isn't a huge problem (could be mistaken). I guess it depends...
In regards to readability, I prefer using dashes (-), as they tend to be easier to read. Underscores may be mistaken for a space). Here's what Matt Cutts had to say about this some years ago: http://www.mattcutts.com/blog/whitehat-seo-tips-for-bloggers/ (and http://www.mattcutts.com/blog/dashes-vs-underscores/)
I believe I have read that Google and other search engines read URLs like this when looking for semantic meanings:
- /this-is-part-of-a-website-address = this is part of a website address
- /this_is_part_of_a_website_address = thisispartofawebsiteaddress
At least that used to be the case...It could be changed now.
In your example, I would not obsess too much about it, as it gives perfect semantic meaning. Have you considered removing special characters, instead of replacing them with a "-" ?
Hope this helps.
Best regards,
Anders