Update contract signatures in README

README.md

@@ -429,12 +429,12 @@ You may need to do custom operations while signing the API request. You can crea
 
 ```php
 // CustomAuthorizationSigner.php
-use GuzzleHttp\Psr7\Request;
+use Psr\Http\Message\RequestInterface;
 use SellingPartnerApi\Contract\AuthorizationSignerContract;
 
 class CustomAuthorizationSigner implements AuthorizationSignerContract
 {
- public function sign(Request $request, Credentials $credentials): Request
+ public function sign(RequestInterface $request, Credentials $credentials): RequestInterface
  {
  // Calculate request signature and request date.
 
@@ -478,17 +478,17 @@ You may also need to customize the entire request signing process – for instan
 
 ```php
 // RemoteRequestSigner.php
-use GuzzleHttp\Psr7\Request;
+use Psr\Http\Message\RequestInterface;
 use SellingPartnerApi\Contract\RequestSignerContract;
 
 class RemoteRequestSigner implements RequestSignerContract
 {
  public function signRequest(
- Request $request,
+ RequestInterface $request,
  ?string $scope = null,
  ?string $restrictedPath = null,
  ?string $operation = null
- ): Request {
+ ): RequestInterface {
  // Sign request by sending HTTP call
  // to external/separate service instance.