Go 1.20+ compiles the example in 09-multiple-generic-types.md that calls for explicit instantiation

[syedriko]

This is the example:

go-generics-the-hard-way/03-getting-started/09-multiple-generic-types.md

Line 74 in 2688bc6

./prog.go:36:26: cannot use generic function Sum without instantiation

And here's the playground link: https://go.dev/play/p/ZeSWNvI-SQi?v=gotip