fmt: correct documentation for Formatter

fmt: correct documentation for Formatter

Before this CL, the documentation for Formatter suggested that
implementers of Format(f State, verb rune) could use Fprint(f) or
Sprint(f) to generate output. The Sprint(f) suggestion however is
invalid.

Fix that by simply suggesting Sprint() alongside Fprint(f).

Fixes #60358

Change-Id: I024e996f6360b812968ef2cd5073cb4c223459e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/497379
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Paul Jolly <paul@myitcv.org.uk>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>