expvar: fix typo in Publish documentation

Found and fixed by bketelsen@gmail.com.
Not worth making him a CONTRIBUTOR to delete one character.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5476054

src/pkg/expvar/expvar.go

@@ -193,7 +193,7 @@ func (f Func) String() string {
 var vars map[string]Var = make(map[string]Var)
 var mutex sync.Mutex
 
-// Publish declares an named exported variable. This should be called from a
+// Publish declares a named exported variable. This should be called from a
 // package's init function when it creates its Vars. If the name is already
 // registered then this will log.Panic.
 func Publish(name string, v Var) {