Fix incorrect array type of rootValue in PHPDocs

(cherry picked from commit 77448ba623)

src/Executor/Executor.php

@@ -119,7 +119,7 @@ class Executor
      *
      * Useful for async PHP platforms.
      *
-     * @param mixed[]|null $rootValue
+     * @param mixed|null   $rootValue
      * @param mixed|null   $contextValue
      * @param mixed[]|null $variableValues
      * @param string|null  $operationName

src/Executor/ReferenceExecutor.php

@@ -115,7 +115,7 @@ class ReferenceExecutor implements ExecutorImplementation
      * Constructs an ExecutionContext object from the arguments passed to
      * execute, which we will pass throughout the other execution methods.
      *
-     * @param mixed[]             $rootValue
+     * @param mixed               $rootValue
      * @param mixed               $contextValue
      * @param mixed[]|Traversable $rawVariableValues
      * @param string|null         $operationName