doc: add missed inspect with numericSeparator to example

In exmple of `util.inspect` with numericSeparator option,
calling `util.inspect` is missed. So actual result is different
from expected result.

doc/api/util.md

@@ -777,8 +777,14 @@ const million = 1_000_000;
 const bigNumber = 123_456_789n;
 const bigDecimal = 1_234.123_45;
 
-console.log(thousand, million, bigNumber, bigDecimal);
-// 1_000 1_000_000 123_456_789n 1_234.123_45
+console.log(inspect(thousand, { numericSeparator: true }));
+// 1_000
+console.log(inspect(million, { numericSeparator: true }));
+// 1_000_000
+console.log(inspect(bigNumber, { numericSeparator: true }));
+// 123_456_789n
+console.log(inspect(bigDecimal, { numericSeparator: true }));
+// 1_234.123_45
 ```
 
 `util.inspect()` is a synchronous method intended for debugging. Its maximum