flag: update comment to refer to Output, not out

The out method was renamed to Output in CL 70391 for #17628 and #21888.

Fixes #37514

Change-Id: I99be47b5030ccbbf10a056df9fcc3c97cb99b015
Reviewed-on: https://go-review.googlesource.com/c/go/+/221383
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

flag.go

@@ -331,7 +331,7 @@ type FlagSet struct {
 	formal        map[string]*Flag
 	args          []string // arguments after flags
 	errorHandling ErrorHandling
-	output        io.Writer // nil means stderr; use out() accessor
+	output        io.Writer // nil means stderr; use Output() accessor
 }
 
 // A Flag represents the state of a flag.