Fix typo in "Solid HTTPS REST API Spec"

[Potherca]

When reading through the specs for the Solid REST-API, I noticed that the section regarding "WAC-Allow headers" contains a typo.

As this typo is not resolved in either #103 nor #148, I felt it best to bring it to your attention.

The offending sentence is:

`user="` + user-permissions = `",public="` + public-permissions + `"`

Unless I am mistaken, given the context, the equals-sign = after user-permissions should be a plus +:

`user="` + user-permissions + `",public="` + public-permissions + `"`

This MR fixes that.