fmt: fix %q verb documentation to say "rune literal"

[Herrtian]

The Go specification uses the term "rune literal" (not "character
literal") for single-quoted literals like 'a'. The %q verb
documentation in the fmt package uses the outdated term "character
literal". This change updates it to use the correct Go terminology.

As noted by @adonovan in #78486, %q works with rune and byte types
but not general int types (which is enforced by go vet since
Go 1.26).

Fixes #78486

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