tour: Clarify the default high bound for a slice

[justdigi]

Context: https://go.dev/tour/moretypes/10

When slicing, you may omit the high or low bounds to use their defaults instead.
The default is zero for the low bound and the length of the slice for the high bound.

Should "length of the slice for the high bound" be length of the array? At least in the example, the variable a is initialized as an array, not a slice.