fix methods return type in exceptions

Author: dvlpr1996

src/exceptions/FileDoesNotExistsException.php

@@ -11,7 +11,7 @@ public function __construct(string $message, int $statusCode = 500, $previous =
         parent::__construct($message, $statusCode, $previous);
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return __CLASS__ . ' : ' . $this->message;
     }

src/exceptions/LanguageFileIsNotArrayException.php

@@ -11,7 +11,7 @@ public function __construct(string $message, int $statusCode = 500, $previous =
         parent::__construct($message, $statusCode, $previous);
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return __CLASS__ . ' : ' . $this->message;
     }

src/exceptions/UrlIsNotValidException.php

@@ -11,7 +11,7 @@ public function __construct(string $message, int $statusCode = 500, $previous =
         parent::__construct($message, $statusCode, $previous);
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return __CLASS__ . ' : ' . $this->message;
     }