doc: add missing changes entry

The `util.format()` behavior changed recently. Add the changes entry
to document the new BigInt behavior.

PR-URL: #24758
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/util.md

@@ -194,6 +194,9 @@ changes:
     pr-url: https://github.com/nodejs/node/pull/17907
     description: The `%o` specifier's `depth` option will now fall back to the
                  default depth.
+  - version: v10.12.0
+    pr-url: https://github.com/nodejs/node/pull/22097
+    description: The `%d` and `%i` specifiers now support BigInt.
   - version: v8.4.0
     pr-url: https://github.com/nodejs/node/pull/14558
     description: The `%o` and `%O` specifiers are supported now.