fmt: fix an error in documentation for fmt

Original Printf("%d", hi) obviously doesn't produce
%!d(string=hi) unless somewhere before this code
block you have hi := "hi" somewhere, also this change
maintains consistency with the rest of it

Change-Id: I40d8cca623176dcad66374ba74e3a1f8f975ac9e
GitHub-Last-Rev: 242e9ee6afba7ab22ed2967b0ba01ef18db01ca9
GitHub-Pull-Request: golang/go#30223
Reviewed-on: https://go-review.googlesource.com/c/162541


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>