Fix typo response_headers in httpx transport (#407)

Author: cybniv

gql/transport/httpx.py

@@ -32,7 +32,7 @@
 class _HTTPXTransport:
     file_classes: Tuple[Type[Any], ...] = (io.IOBase,)
 
-    reponse_headers: Optional[httpx.Headers] = None
+    response_headers: Optional[httpx.Headers] = None
 
     def __init__(
         self,