BigDecimal doc link fixup

Author: sunny

README.md

@@ -489,7 +489,7 @@ You can set the default rounding mode by passing one of the `BigDecimal` mode en
 Money.rounding_mode = BigDecimal::ROUND_HALF_EVEN
 ```
 
-See [BigDecimal::ROUND_MODE](https://ruby-doc.org/stdlib-2.5.1/libdoc/bigdecimal/rdoc/BigDecimal.html#ROUND_MODE) for more information.
+See [BigDecimal::ROUND_MODE](https://ruby-doc.org/3.4.1/gems/bigdecimal/BigDecimal.html#ROUND_MODE) for more information.
 
 To round to the nearest cash value in currencies without small denominations: