expvar: fix name of Var interface

Change-Id: Ibc40237981fdd20316f73f7f6f3dfa918dd0af5d
Reviewed-on: https://go-review.googlesource.com/133658Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/expvar/expvar.go

@@ -221,7 +221,7 @@ func (v *String) Value() string {
 	return p
 }
 
-// String implements the Val interface. To get the unquoted string
+// String implements the Var interface. To get the unquoted string
 // use Value.
 func (v *String) String() string {
 	s := v.Value()