Fix an error message to be more accurate

Author: deepjyoti30-st

v2/thirdpartyemailpassword/common-customizations/handling-signinup-success.mdx

@@ -599,7 +599,7 @@ func main() {
 									if field.ID == "name" {
 										valueAsString, asStrOk := field.Value.(string)
 										if !asStrOk {
-											return epmodels.SignUpPOSTResponse{}, errors.New("Should never come here as we check the type during validation")
+											return epmodels.SignUpPOSTResponse{}, errors.New("name should be a string")
 										}
 										name = valueAsString
 									}