README: fix broken URL

The reference URL for Supported m.room.message(msgtypes) used in fractal, was broken. Each time the current URL is clicked, we get the error:The requested URL was not found on this server This URL directs us to Each m.room.message(msgtypes) and their msgtype key which identifies the type of message being sent. Each type has their own required and optional keys which can be seen when u access that page. Now, what makes this URL broken is the \ in client\_server once the \ is removed , it is then possible to access the URL and see the above mentioned.