let AsyncPipe::create return the correct type

Summary: Let `AsyncPipe::create` return the correct pipe type, namely, the type on which it is a member rather than a different instantiation of the template.

Reviewed By: iahs

Differential Revision: D25717180

fbshipit-source-id: 94a5a657e83b5d4cac866277ec5925c0b5b0af28

folly/experimental/coro/AsyncPipe.h

@@ -80,7 +80,7 @@ class AsyncPipe {
     return *this;
   }
 
-  static std::pair<folly::coro::AsyncGenerator<T&&>, AsyncPipe<T>> create(
+  static std::pair<folly::coro::AsyncGenerator<T&&>, AsyncPipe> create(
       folly::Function<void()> onClosed = nullptr) {
     auto queue = std::make_shared<Queue>();
     auto cancellationSource = std::shared_ptr<folly::CancellationSource>();