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

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documentation in the fmt package uses the outdated term "character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

Change-Id: I894f339d6242331f8b02df3cf3d432df41e50772
GitHub-Last-Rev: 1f65e43d3b8c1f002c3676499b229bae3a8deac6
GitHub-Pull-Request: golang/go#78521
Reviewed-on: https://go-review.googlesource.com/c/go/+/762580
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Alex Putman <aputman@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
diff --git a/src/fmt/doc.go b/src/fmt/doc.go
index 49365ee..a6724e6 100644
--- a/src/fmt/doc.go
+++ b/src/fmt/doc.go
@@ -43,7 +43,7 @@
 	%d	base 10
 	%o	base 8
 	%O	base 8 with 0o prefix
-	%q	a single-quoted character literal safely escaped with Go syntax.
+	%q	a single-quoted rune literal safely escaped with Go syntax.
 	%x	base 16, with lower-case letters for a-f
 	%X	base 16, with upper-case letters for A-F
 	%U	Unicode format: U+1234; same as "U+%04X"