Fix static analysis error

Error message:
    ERROR: MethodSignatureMustProvideReturnType -
    ../src/Curl/Url.php:18:21 - Method Curl\Url::__toString must have a
    return type signature! (see https://psalm.dev/282) public function
    __toString()

src/Curl/Url.php

@@ -15,7 +15,7 @@ class Url
         $this->relativeUrl = $relative_url;
     }
 
-    public function __toString()
+    public function __toString() : string
     {
         return $this->absolutizeUrl();
     }