doc/reference-cmd: use vet as example, not fmt

"go tool fmt" doesn't work, "go tool vet" does.

R=golang-dev, rogpeppe, r, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/5900049

doc/reference-cmd.html

@@ -19,10 +19,10 @@ with arguments appropriate to package-level processing.
 
 <p>
 The programs can also be run as stand-alone binaries, with unmodified arguments,
-using the go tool subcommand, such as "go tool fmt".
-This style of invocation allows, for instance, reformatting a single source file rather than
-an entire package: "go tool fmt myprogram.go" as compared to
-"go fmt mypackage".
+using the go tool subcommand, such as "go tool vet".
+This style of invocation allows, for instance, checking a single source file rather than
+an entire package: "go tool vet myprogram.go" as compared to
+"go vet mypackage".
 Some of the commands, such as prof and yacc, are accessible
 only through the go tool subcommand.
 </p>