ComplexityLimit increments one point for Scalars as well

Hi,

Thank you for this amazing library, and all the hard work put into making this. It definitely makes my day-to-day life easier

### What happened?
- I used extenstion.FixedLimitComplexity(5)
### What did you expect?
- I expected the following behavior
```
query {
shop { # Object - 1 point
id # ID - 0 points
name # String - 0 points
timezoneOffsetMinutes # Int - 0 points
customerAccounts # Enum - 0 points
}
}
```
https://shopify.engineering/rate-limiting-graphql-apis-calculating-query-complexity

However, I noticed that the complexity is incremented by 1 even for scalar types. [This](https://github.com/99designs/gqlgen/blob/master/complexity/complexity.go#L45) is the line that is responsible for it.

If this is not the expected behavior, I'm happy to make the change to fix the issue. Please let me know.

Thanks