[CHORE] Fix swagger deprecation message

- Fix "WARNING: item list for enum is not a valid JSON array, using the
old deprecated format" messages from
https://github.com/go-swagger/go-swagger in the CI.

modules/structs/repo_collaborator.go

@@ -5,7 +5,7 @@ package structs
 
 // AddCollaboratorOption options when adding a user as a collaborator of a repository
 type AddCollaboratorOption struct {
-	// enum: read,write,admin
+	// enum: ["read", "write", "admin"]
 	Permission *string `json:"permission"`
 }