Remove unnecessary mixed from declaration

Author: bwoebi

README.md

@@ -56,7 +56,7 @@ interface Promise
      *
      * If the promise is already resolved, the callback MUST be executed immediately.
      *
-     * @param callable(\Throwable|\Exception|null $exception, mixed $value) @onResolved `$reason` shall be `null` on
+     * @param callable(\Throwable|\Exception|null $exception, $value) @onResolved `$reason` shall be `null` on
      *     success, `$value` shall be `null` on failure.
      *
      * @return mixed Return type and value are unspecified.

src/Promise.php

@@ -12,10 +12,10 @@ interface Promise
      *
      * If the promise is already resolved, the callback MUST be executed immediately.
      *
-     * @param callable(\Throwable|\Exception|null $exception, mixed $value) @onResolved `$reason` shall be `null` on
+     * @param callable(\Throwable|\Exception|null $exception, $value) @onResolved `$reason` shall be `null` on
      *     success, `$value` shall be `null` on failure.
      *
      * @return mixed Return type and value are unspecified.
      */
     public function when(callable $onResolved);
-}
+}