made tutorial outputs more presentable (#555)

Closes https://github.com/ooni/probe/issues/1831

internal/tutorial/measurex/chapter08/README.md

@@ -111,7 +111,7 @@ This is it. The rest of the program is exactly the same.
 Let us perform a vanilla run first:
 
 ```bash
-go run -race ./internal/tutorial/measurex/chapter08
+go run -race ./internal/tutorial/measurex/chapter08 | jq
 ```
 
 Please, check the JSON output. Do you recognize the fields

internal/tutorial/measurex/chapter08/main.go

@@ -112,7 +112,7 @@ func main() {
 // Let us perform a vanilla run first:
 //
 // ```bash
-// go run -race ./internal/tutorial/measurex/chapter08
+// go run -race ./internal/tutorial/measurex/chapter08 | jq
 // ```
 //
 // Please, check the JSON output. Do you recognize the fields