cmd/go: fix typo in error message

Change-Id: I9b618de68a5f2c89caae06d35a153cda2f466b6a
Reviewed-on: https://go-review.googlesource.com/78678Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/generate/generate.go

@@ -385,7 +385,7 @@ func (g *Generator) setShorthand(words []string) {
 	}
 	command := words[1]
 	if g.commands[command] != nil {
-		g.errorf("command %q defined multiply defined", command)
+		g.errorf("command %q multiply defined", command)
 	}
 	g.commands[command] = words[2:len(words):len(words)] // force later append to make copy
 }