Add return type mixed to offsetGet

Turns out that #175 was not enough to make Symfony happy:
```
Method "ArrayAccess::offsetGet()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "FriendsOfBehat\SymfonyExtension\Mink\MinkParameters" now to avoid errors or add an explicit @return annotation to suppress this message.
```

src/Mink/MinkParameters.php

@@ -27,6 +27,9 @@ class MinkParameters implements \Countable, \IteratorAggregate, \ArrayAccess
         return array_key_exists($offset, $this->minkParameters);
     }
 
+    /**
+     * @return mixed
+     */
     #[ReturnTypeWillChange]
     public function offsetGet($offset)
     {