Fixed issue with PHP 7.4 where cssdoc::offsetGet() had an argument with the wrong type.

Author: hexydec

src/cssdoc.php

@@ -284,7 +284,7 @@ public function offsetUnset($i) : void {
 	 * @param string|integer $i The key to be accessed, can be a string or integer
 	 * @return mixed The requested value or null if the key doesn't exist
 	 */
-	public function offsetGet(mixed $i) : mixed { // return reference so you can set it like an array
+	public function offsetGet($i) : mixed { // return reference so you can set it like an array
 		return $this->document->rules[$i] ?? null;
 	}