[fidl][go] Fix remaining union test issue when go flag is flipped

Change-Id: I9998717aea4adf2f24300d0d81a0b7d1e8a1b4b9
diff --git a/src/syscall/zx/fidl/encoding_new.go b/src/syscall/zx/fidl/encoding_new.go
index d71f256..058c5a7 100644
--- a/src/syscall/zx/fidl/encoding_new.go
+++ b/src/syscall/zx/fidl/encoding_new.go
@@ -777,7 +777,7 @@
 		// different from the xunion ordinal if there are reserved variants.
 		fieldIndex := int(v.Field(0).Uint())
 		if !(fieldIndex >= 1 && fieldIndex <= len(m.ordinals)) {
-			return newValueError(ErrInvalidUnionTag, fieldIndex)
+			return newValueError(ErrInvalidXUnionTag, fieldIndex)
 		}
 		ordinal = m.ordinals[fieldIndex-1]
 	} else {