cmd/doc: fix build

Change-Id: Ic8437a1d2aeb424d6d5ce9e608c1293bba4c7bbc
Reviewed-on: https://go-review.googlesource.com/10093
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/doc/pkg.go

@@ -370,7 +370,7 @@ func (pkg *Package) symbolDoc(symbol string) {
 // structs and methods from interfaces (unless the unexported flag is set).
 func trimUnexportedElems(spec *ast.TypeSpec) {
 	if *unexported {
-		return fields
+		return
 	}
 	switch typ := spec.Type.(type) {
 	case *ast.StructType: