Fix a typo inside handlers_intro.md (#1919)

axum/src/docs/handlers_intro.md

@@ -2,7 +2,7 @@ In axum a "handler" is an async function that accepts zero or more
 ["extractors"](crate::extract) as arguments and returns something that
 can be converted [into a response](crate::response).
 
-Handlers is where your application logic lives and axum applications are built
+Handlers are where your application logic lives and axum applications are built
 by routing between handlers.
 
 [`debug_handler`]: https://docs.rs/axum-macros/latest/axum_macros/attr.debug_handler.html