tour: character removed in deference to rune

[crisman]

In tour/basics/15 ("Constants") the word "character" is both new (not used in the tour before this slide) and confusing as it is not the Go way to talk about runes. Since "numeric values" is called out a few words later (which includes rune constants), neither rune nor character is needed here.

Remove odd word "character" to avoid confusing readers with unneeded non-Go terminology.

Fixes golang/tour#497 Fixes golang/tour#1151

[gopherbot]

This PR (HEAD: 75f76a967531dd2d6a76fb6ab1320a569801f90f) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/website/+/473055 to see it.

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