isogram: "invert if" case not caught

```go
package isogram

import "unicode"

// IsIsogram takes a string as input and decides whether it's an isogram or not
func IsIsogram(s string) bool {
m := make(map[rune]int, len(s))
for _, r := range s {
if unicode.Is(unicode.Letter, r) {
if _, ok := m[unicode.ToLower(r)]; ok {
return false
}
m[unicode.ToLower(r)]++
}
}
return true
}
```

This should show a comment about inverting the `if` for happy path on the left.