[Config] Very minor grammar fix in error message

It's just a tiny detail, but I found it important enough to submit a fix.

| Q   | A
| --- | ---
| License | MIT
| Fixed tickets | -
I would change the order of the comparison to follow Symfony common practices (`1 == count($value)`), I'd change `==` by `===` and I'd use `''` instead of `null` for the `1` case.