Clarify what handler::any and service::any accepts (#337)

They only accept standard HTTP methods and don't think we can fix that
without breaking changes.

Fixes https://github.com/tokio-rs/axum/issues/289

src/routing/method_filter.rs

@@ -16,7 +16,7 @@ bitflags! {
         const OPTIONS = 0b000010000;
         /// Match `PATCH` requests.
         const PATCH =   0b000100000;
-        /// Match `POSt` requests.
+        /// Match `POST` requests.
         const POST =    0b001000000;
         /// Match `PUT` requests.
         const PUT =     0b010000000;