Changing a swap in README.md for autobahn

Autobahn test suite configuration fuzzingclient.json refers to 'clients' folder, but README.md refers to server. I changed README.md to clients, although I can't figure a good way to choose between changing README.md for 'clients' or fuzzingclient.json for servers. both seems legit to me, depending on what side you see things. Feel free to dismiss/decline my PR if you think the former makes more sense in this context.