Update custom url schemes error message to reflect what we actually support (#9061)

model/config.go

@@ -2418,7 +2418,7 @@ func (mes *MessageExportSettings) isValid(fs FileSettings) *AppError {
 
 func (ds *DisplaySettings) isValid() *AppError {
 	if len(*ds.CustomUrlSchemes) != 0 {
-		validProtocolPattern := regexp.MustCompile(`(?i)^\s*[a-z][a-z0-9+.-]*\s*$`)
+		validProtocolPattern := regexp.MustCompile(`(?i)^\s*[a-z][a-z0-9-]*\s*$`)
 
 		for _, scheme := range *ds.CustomUrlSchemes {
 			if !validProtocolPattern.MatchString(scheme) {