text/template: fix type of ComplexZero in test

Was stupidly float64; should be complex128.
The tests still pass.

Fixes #5649.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12107044

src/pkg/text/template/exec_test.go

@@ -24,7 +24,7 @@ type T struct {
 	U16         uint16
 	X           string
 	FloatZero   float64
-	ComplexZero float64
+	ComplexZero complex128
 	// Nested structs.
 	U *U
 	// Struct with String method.