Update UPGRADING [skip ci]

Author: DanielEScherzer

UPGRADING

@@ -266,8 +266,8 @@ PHP 8.5 UPGRADE NOTES
     be used.
     RFC: https://wiki.php.net/rfc/deprecations_php_8_4
   . Trying to produce output (e.g. with `echo`) within a user output handler
-    is deprecated. The deprecation warning will bypass the handler with the bad
-    return to ensure it is visible; if there are nested output handlers the next
+    is deprecated. The deprecation warning will bypass the handler producing the
+    output to ensure it is visible; if there are nested output handlers the next
     one will still be used. If a user output handler returns a non-string and
     produces output, a single combined deprecation message is used.
     RFC: https://wiki.php.net/rfc/deprecations_php_8_4