cmd/stringer: tweak "Code generated by" comment to match new standard

See https://golang.org/issue/13560 for the full discussion.

The actual change is just the addition of a final period.

Update golang/go#13560

Change-Id: Icc2f52b67181de344aa5107f94faa0e739ff993c
Reviewed-on: https://go-review.googlesource.com/38415
Reviewed-by: Ian Lance Taylor <iant@golang.org>

stringer.go

@@ -126,7 +126,7 @@ func main() {
 	}
 
 	// Print the header and package clause.
-	g.Printf("// Code generated by \"stringer %s\"; DO NOT EDIT\n", strings.Join(os.Args[1:], " "))
+	g.Printf("// Code generated by \"stringer %s\"; DO NOT EDIT.\n", strings.Join(os.Args[1:], " "))
 	g.Printf("\n")
 	g.Printf("package %s", g.pkg.name)
 	g.Printf("\n")