cmd/vet: fix typo in documentation

familiy -> family

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5783061

src/cmd/vet/doc.go

@@ -13,7 +13,7 @@ Available checks:
 
 1. Printf family
 
-Suspicious calls to functions in the Printf familiy, including any functions
+Suspicious calls to functions in the Printf family, including any functions
 with these names:
 	Print Printf Println
 	Fprint Fprintf Fprintln