Fix new bug81726.phpt for PHP 8.0

The error message has slightly changed, so we adapt our expectations.

Closes GH-9621.

Author: cmb69

ext/phar/tests/bug81726.phpt

@@ -10,5 +10,5 @@ if (!extension_loaded("zlib")) die("skip zlib extension not available");
 var_dump(fopen("phar://" . __DIR__ . "/bug81726.gz", "r"));
 ?>
 --EXPECTF--
-Warning: fopen(phar://%s): failed to open stream: unable to decompress gzipped phar archive "%s" in %s on line %d
+Warning: fopen(phar://%s): Failed to open stream: unable to decompress gzipped phar archive "%s" in %s on line %d
 bool(false)