Fix typo in UPGRADING

[ci skip]

Author: nikic

UPGRADING

@@ -271,7 +271,7 @@ PHP 7.4 UPGRADE NOTES
   . Added mbstring.regex_retry_limit ini setting defaulting to 1000000. It
     limits the amount of backtracking that may be performed during one mbregex
     match and thus protects against exponential backtracking attacks (ReDOS).
-    This setting only takes effect when linking against oniguruma >= 2.8.0.
+    This setting only takes effect when linking against oniguruma >= 6.8.0.
 
 - OPcache:
   . Support for preloading code has been added.