Fix doctests

src/lib.rs

@@ -257,7 +257,7 @@
 //!
 //! ```no_run
 //! # #[cfg(feature = "native-tls")]
-//! # fn build() -> std::result::Result<(), ureq::Error> {
+//! # fn build() -> std::result::Result<(), Box<dyn std::error::Error>> {
 //! # ureq::is_test(true);
 //!   use std::sync::Arc;
 //!   use ureq::Agent;