TYPO: Fixed variable name and position in interface definition. (#154)

I was also confused about the variable names (#153). But in this [commit](5995653) it is clear that the first var is output, second var is input. In this commit I have fixed the issue. Fixes #153

graphql/client.go

@@ -42,7 +42,7 @@ type Client interface {
 		ctx context.Context,
 		opName string,
 		query string,
-		input, retval interface{},
+		retval, input interface{},
 	) error
 }