[haskell] fix exampe code in generated README.md (#16142)

* [haskell] fix exampe code in generated README.md

Since create{{title}}Client is not a monadic action, we need to use
"let" instead of "<-".

* regenerate samples

Author: msakai

modules/openapi-generator/src/main/resources/haskell-servant/README.mustache

@@ -50,7 +50,7 @@ main = do
   manager <- newManager tlsManagerSettings
 
   -- Create the client (all endpoint functions will be available)
-  {{title}}Backend{..} <- API.create{{title}}Client
+  let {{title}}Backend{..} = API.create{{title}}Client
 
   -- Any {{title}} API call can go here, e.g. here we call `getSomeEndpoint`
   API.call{{title}} (mkClientEnv manager url) getSomeEndpoint

samples/server/petstore/haskell-servant/README.md

@@ -50,7 +50,7 @@ main = do
   manager <- newManager tlsManagerSettings
 
   -- Create the client (all endpoint functions will be available)
-  OpenAPIPetstoreBackend{..} <- API.createOpenAPIPetstoreClient
+  let OpenAPIPetstoreBackend{..} = API.createOpenAPIPetstoreClient
 
   -- Any OpenAPIPetstore API call can go here, e.g. here we call `getSomeEndpoint`
   API.callOpenAPIPetstore (mkClientEnv manager url) getSomeEndpoint