[generics] tiny typo in Map/Reduce/Filter examples

[Mark]

At the end of this section where it shows the outputs:

```go

floats := slices.Map(s, func(i int) float64 { return float64(i) })
// Now float2 is []float64{1.0, 2.0, 3.0}.
```
shouldn't that be:
```go
floats := slices.Map(s, func(i int) float64 { return float64(i) })
// Now floats is []float64{1.0, 2.0, 3.0}.
```

[Ian Lance Taylor]

Thanks, sent https://golang.org/cl/239157 to fix this.

Ian