Correct @return in Any.unpack docblock (#7089)

As per https://docs.phpdoc.org/latest/references/phpdoc/tags/return.html, the first token after the @return tag should be the return type.

php/src/Google/Protobuf/Any.php

diff --git a/php/src/Google/Protobuf/Any.php b/php/src/Google/Protobuf/Any.php
index 6c6924c..166c66a 100644
--- a/php/src/Google/Protobuf/Any.php
+++ b/php/src/Google/Protobuf/Any.php
@@ -251,7 +251,7 @@
      * targeted message type. If failed, an error will be thrown. Otherwise,
      * the method will create a message of the targeted type and fill it with
      * the decoded value in Any.
-     * @return unpacked message
+     * @return Message unpacked message
      * @throws \Exception Type url needs to be type.googleapis.com/fully-qualified.
      * @throws \Exception Class hasn't been added to descriptor pool.
      * @throws \Exception cannot decode data in value field.