fix actix-service/src/ext.rs 'fist' to 'first' (#596)

Co-authored-by: yinqilei <yinqilei_ug@126.com>

actix-service/src/ext.rs

@@ -44,7 +44,7 @@ pub trait ServiceExt<Req>: Service<Req> {
     /// Call another service after call to this one has resolved successfully.
     ///
     /// This function can be used to chain two services together and ensure that the second service
-    /// isn't called until call to the fist service have finished. Result of the call to the first
+    /// isn't called until call to the first service have finished. Result of the call to the first
     /// service is used as an input parameter for the second service's call.
     ///
     /// Note that this function consumes the receiving service and returns a wrapped version of it.