Update RawColumn.php (#22)

Phpstan on 5 level give error, because in RawColumn DocBlock is inconsistent with parameters given when calling RawColumn class in code
ex. new RawColumn("sspName", "ssp") cause this error:
"Parameter #2 $alias of class PhpClickHouseLaravel\RawColumn constructor expects null, string given."

src/RawColumn.php

@@ -12,7 +12,7 @@ class RawColumn extends Expression
      * Create a new raw query expression.
      *
      * @param mixed $value
-     * @param null $alias
+     * @param string|null $alias
      */
     public function __construct($value, $alias = null)
     {