avoid MSVC warning via flag_if_supported
diff --git a/src/lib.rs b/src/lib.rs
index 78aba3d..7e6ece4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -233,7 +233,7 @@
 //! fn main() {
 //!     cxx_build::bridge("src/main.rs")  // returns a cc::Build
 //!         .file("../demo-cxx/demo.cc")
-//!         .flag("-std=c++11")
+//!         .flag_if_supported("-std=c++11")
 //!         .compile("cxxbridge-demo");
 //!
 //!     println!("cargo:rerun-if-changed=src/main.rs");